In fact a recent study came out showing that more than half of the youth of the Basque Country are neolocutors, meaning they did not grow up speaking Basque at home but learned it in school or elsewhere.
The results of the report compiled from 2016 data can be read in this article.
A point of clarification. There's a difference between a native bilingual, who learned for example Basque and Spanish at home at the same time, and a Basque native speaker who only learned Basque at home and started learning Spanish or French outside the home. Eventually these Basque native speakers end up learning erdara (foreign language) and do become bilingual, but this survey differentiates between them based on their home language(s). So:
1) native Basque speakers
2) native bilingual speakers
3) new speakers who only grew up with erdara but became bilingual later in life (at school for example).
Let's see some figures.
Euskal Herriko euskaldunen %50,2 dira euskaldun zaharrak gaur egun; alegia, euskara etxean ikasi dutenak. Jatorrizko elebidunak —etxean euskara eta beste hizkuntza bat jaso dituztenak— %13,6 dira, eta euskaldun berriak, %36,2 —herenak baino gehiago—.
Datua are nabarmenagoa da gazteei erreparatuz gero: 2016an, 16 eta 24 urte bitarteko gazte euskaldunen artean %29,8 baino ez ziren euskaldun zaharrak, %15,9 jatorrizko elebidunak, eta %54,3 euskaldun berriak. Gazteen erdiak baino gehiago euskaldun berriak dira, beraz.
In the Basque Country, 50.2% of Basque speakers are native speakers. 13.6% are native bilinguals, having learned both Basque and another language at home. New speakers who learned no Basque at home but learned it later are 36.2% of the population.
It gets more interesting when you look at the youth between the ages of 16-24.
29.8% are native Basque speakers.
15.9% are native bilinguals.
54.3% are new speakers of Basque.
Most of the article however is not dedicated to the results per se but the categories used in the classification of Basque speakers, euskaldun berriak (new speakers, those who learned Basque outside the home and are not native speakers) and euskaldun zaharrak (literally old speakers, those who learned Basque at home and are native speakers). Many criticise this separation as essentialist, and even within the scientific context can come with ideological weight, like the idea that the native speakers are better speakers (solely for having learned it at home) than those who learned it in some other way.
I think it's questionable whether this classification is pertinent in all cases. I'm not saying it's not pertinent in some cases and a useful term, but there are situations in which you could group both a euskaldun berria and zaharra together if their language behaviours overlap.
For example does it matter who is a native speaker or a new learner, when you're interested in quantifying who uses the language more and it turns out some new learners use Basque much more than the native speaker?
That doesn't mean there aren't important differences to note between native speakers and new learners, but they have to do with the use of the language (the study doesn't concern itself with 'quality' of the language):
Erabilera handiagoa dute lehenengo hizkuntza euskara duten hiztunek, lehenengo hizkuntza euskara ez dutenek baino». Hori erakusten du inkesta soziolinguistikoak erabilerari begiratuta, adibidez: euskaldun zaharren %84,1ek egiten dute hizkuntzaren erabilera trinkoa —alegia, erdaraz adina edo gehiago egiten dute euskaraz—, eta euskaldun berrien artean, berriz, %22,9 dira euskara erabili ohi dutenak. Gaitasun erlatiboan ere badago aldea: euskaldun zaharren artean, %47,2k errazago egiten dute euskaraz, eta euskaldun berrien artean, %83,1ek errazago erdaraz. Erraztasuna eta erabilera loturik daude, dena den; baita lehen hizkuntza ere.
What this says is that 84.1% of native Basque speakers speak more Basque than erdara (Spanish/French) in their daily life, compared to 22.9% of new Basque speakers.
As for their self declared ease or capacity in the language, 47.2% of native speakers say that they find Basque easier to speak than erdara, whereas 83.1% of new speakers say that they feel erdara is easier for them than Basque.
You might be tempted to go at this point 'ahah! Native speakers feel that speaking Basque is easier for them, that proves there's an inherent difference' but I'm not so sure. A part of the ease of speech comes from native acquisition, sure. But a big part also comes from how frequently you use the language, which explains those cases of native attrition. We all know stories of people who go live in another country for 20 years without needing to speak the language and they lose much of their ease in their native language, and at that point, you could pit them up with a new speaker who uses the new language every day, and the new speaker could come out ahead on their self identified fluency.
At this point it's worth considering another factor that is too overlooked in favour of the abstract, all powerful 'native language' (as if your first language will determine your language skills and habits for your entire life!). And that is your environment:
Eta gune soziolinguistikoei erreparatuta, argi ikusten da: euskaldun zahar, berri zein jatorrizko elebidunen erabilera anitz igotzen da euskaldunak anitz diren herrietan. Amorrortuk uste du horri erreparatu behar zaiola gehiago, eta ez hainbertze lehen hizkuntzari: «Aldagai garrantzitsu bat, euskararen kasuan, testuinguruarena da».
Native speakers by definition have a space for use that new speakers don't, which is the family. So they immediately have a benefit there. But, there are other contexts in which the language is used. The key information in this quote is:
Whether you are a native Basque speaker, a native bilingual speaker or a new speaker, all of their usage of Basque goes up in towns where Basque is widely spoken.
There's nothing written in the laws of nature that says that new speakers, because they grew up with erdara, inherently must feel uncomfortable with Basque or make lesser use of it.
The 83.1% of new speakers who say that they feel they speak Spanish/French easier than Basque, feel that way at least in part because their context (including family but also school, friends, extracurricular activities etc) is so heavily biased towards erdara.
How big of a part context plays is a very important question. Even if it were true that your native language determines in a magical way your language skills and habits forever, it is little help in saving the Basque language. If we could immediately ensure everyone born in the Basque Country from the next five seconds onwards had committed parents who would speak to them at home in Basque, that would be fantastic! But it's hard to do. It's hard to create native speakers for a minoritised language.
Whereas knowing that the context of use determines your language skills and use of said language, can be targeted with concrete actions here and now. Such as promoting Basque in the public space, in administration, health care, education etc: all this not only helps new speakers so that their use and fluency rises to match that of native speakers, it helps native speakers too. After all what's the point of being a native speaker if you never use it or have an opportunity to use it?
Final thing. The journalist who wrote this article is
Maddi Ane Txoperena Iribarren, born in Hendaia, Lapurdi. She writes in a North Basque Batua in this article (bertze, -en suffix for future marking etc). These are all forms accepted in the umbrella of Batua, and the author depending on their providence can choose to write in them. Berria, the Basque newspaper allows for this variety, so from one article to the next you can literally read different forms of standard Basque.
It'd be like a pan-German newspaper allowing the use of forms and vocabulary from standard German German, standard Austrian German, standard Swiss German etc.