Oscar whistles "Merry" and then says "Christmas" afterwards. He has all kinds of whistles, not including the "Merry". lol
For age, I don't know if someone already mentioned this, but look at his eyes. If he's an adult, they'll be a pale yellow. If he's pretty young still, they'll be grey. Plus, if he's really young, he'll have black on the tips of his tail feathers.
As for clipping his wings, I'm sure the vet could trim them if you wanted him/her to. Otherwise he/she'll use a towel. You'll probably be brought into a back room so you won't have to worry about him flying away.