Voice changes are very common so don't worry. It's a big part of what signifies that you're growing into an 'adult' (though I use the word adult loosely) As for your DSi, I don't think anyone under 20 is too old for one. I mean it's part of having fun. You see people playing on their cell phones who are in their 40s. Having fun and playing games isn't something limited to children. However, donating it to a younger kids who may have never gotten one if it weren't because of your generosity, is a sign of great maturity from your part. Plus, now you get to upgrade to an xbox
I'm 21 and play with 360 every day, with people who are grown and have kids of their own. Like I said, games are for everyone.