Search through the list archives...there are several threads on young hikers. My son was 13 when we did Whitney in 2001 (see trip report under "highpointing" on my <a href="http://www.mtritter.org">WWW site</a>). Kids several years younger than that have made it, usually as multi-day backpacking trips like we did, not dayhikes.
When we were up there in '01, we saw a German family on the summit with some kids that looked to be in the 8-10-year range, plus a 9-month-to-1-year-old in one of those baby backpack carriers. I commented at the time that there was no way I'd ever consider taking a pre-verbal infant that high...kid couldn't tell you if he/she had a headache or other signs if developing AMS. Not a good idea...
That said, my son started backpacking with me when he was about 5, and now that he's 15, he can be my pack horse and shoulder more than 50% of the load...make up for all those times he carried 10# and I carried 60#...