The literature exploring the benefits of learning a second language at a young age is quite extensive. The American Council for the Teaching of Foreign Languages (ACTFL) maintains a resource page with references to different research studies highlighting the benefits of bilingualism.
Here's the link: https://www.actfl.org/advocacy/what-the-research-shows
Building on earlier evidence showing a beneficial effect of bilingualism on children's cognitive development, we review recent studies using both behavioral and neuroimaging methods to examine the effects of bilingualism on cognition in adulthood and explore possible mechanisms for these effects. This research shows that bilingualism has a somewhat muted effect in adulthood but a larger role in older age, protecting against cognitive decline, a concept known as ‘cognitive reserve’. We discuss recent evidence that bilingualism is associated with a delay in the onset of symptoms of dementia. Cognitive reserve is a crucial research area in the context of an aging population; the possibility that bilingualism contributes to cognitive reserve is therefore of growing importance as populations become increasingly diverse.
Bilingualism: consequences for mind and brain
Bialystok, Ellen et al.
Trends in Cognitive Sciences , Volume 16 , Issue 4 , 240 - 250