I think my older daughter was somewhere around 2.5. I tried closer to 2, but she had no interest in it and was kind of grossed out by it. At almost 4, she's obsessed now!
I want to start letting my 1.5 year old do some "arts and crafts" with us but she samples everythingggg, so play doh will be awhile out for her, I think. Unless I make some of the edible stuff, but I don't know which recipes work!