I love my steam deck and I think the summer sale is a great deal for it. I would definitely recommend it, to try and alleviate your worries, it is confirmed that the steam deck two will not be out for at least another 4 years. Also, the other commenter said something about the lifespan lasting a short amount of time, that is highly unlikely. It can run almost any PC game, and there is no way that Valve won’t continue to update SteamOS; on the off chance that they did stop, you could switch to a different Linux distro anyway, but like I said it is close to impossible that that would happen.
Being malnourished as a child/teen (pretty much any time prior to or at the beginning of puberty) can stunt growth. It can also cause other issues, but sometimes these children will grow into their 20s during a phase called “catch up growth”.