Do you let your kids drink soda and how old are they?

My 2 year old son had his first taste of Dr. Pepper today and it wasn't because I gave it to him - I left it on the end table when I went to the restroom. I came out and saw him put the can on the table and let out a big 'Aaaah', apparently he found out why mommy drinks it LOL While talking to a friend this morning and telling her about it she told me that her niece, who just turned 4, drinks nothing but soda - I was a little shocked to say the least. So just out of curiousity what age did you let your child have their first soda? How old are they now and how much do they drink? Thanks! :)