Instead of recommending an age, let me share somethings I've learned:
1) my 5 year old still hasn't used the SAK Tinker I bought for him when he was 6 months old
- he's not ready to carry it on his own, so we use my knife for most things.
2) he's used the little scissors on 58 mm SAKs more than anything else, and loves it.
- the Jetsetter was great; it's too bad he lost it already.
- he did once drop it on his bare foot and make a small puncture with the scissor tip. Not a huge deal, but worth noting that even the little scissors can draw blood.
3) He doesn't have any real use yet for an actual blade
- cutting vegetables isn't fun because he can't do it anyway he wants to
- whittling anything besides a pointy stick is probably beyond his attention span
- if he's cutting anything else, I probably want to know about it.
4) He doesn't demonstrate good awareness of where the blade tip is pointing. I used to think the rounded tipped knives were silly, but I've come around. He's got a "my first Opinel" no. 7 now, which will be the training folding knife. I'm going to let him start using it in 2 weeks.
your mileage may vary, but I have realized that my desire for him to have a knife is unrealistically greater than the number of uses a 5 year old has for one. Now that said, I think exposure is a key part of education, and knowing he's got special knives set aside for when he's ready is a great motivator. We'll keep using them together, supervised, trying to train some discipline into him.
I've really come to appreciate scissors as a good training tool, and honestly for the past two years he's been mostly interested in cutting paper into fun shapes. As far as I'm concerned that's a good start.