@ Wataya: I've never seen or heard of "youngs" being used like that (your web example). For what it is worth, I am a forest technician by education, a forest insect and disease technician by occupation, with my former occupation being a forest and wildlife technician with Parks Canada.
Are you sure? Random internet sites may not be the best evidence, but still: http://www.myuniversalfacts.com/2006/04/names-of-animals-and-their-youngs.html I guess: 'the young' as the plural within a species and 'the youngs' as the plural among species.
When used as a noun, the plural of "young" is "young". It is one of those English words that doesn't change in the plural form. I would suggest the Doulingo hint is mistaken. The German translation I found for "the young" (plural) was "die Jungen". I think the only real translation here is "his boys". (Feel free to correct me my German friends). His young would be "seine Jungen" maybe?
There is one additional interpretation: in colloquial speech 'die Jungs' can refer to a group of (not necessarily young) males you are very well aquainted with. Like in 'Ich spiele Poker mit den Jungs' - 'I play poker with the guys'. But this is very colloquial and IMHO duo shouldn't teach it.