Is it illegal to buy alcohol if you are under 21, or just to sell it?

If someone under the age of 21 attempts to purchase alcohol, can they get into any legal trouble, or is it only illegal for the clerk to sell it? If you can get into trouble for trying to buy it, or for buying it, what are the minimum and maximum punishments. I live in New York, but I’m curious about other places as well.

I don’t smoke, but just out of curiosity, how about tobacco products?