In reply to VPJB:
Outdoor bouldering - before they could walk
Roped climbing - they were interested from about 5 (smaller one now 4.5 and beginning to be interested). At 7 (and a friend's kid at 6) big one is looking very composed on the rock, calmly making his way up easy short routes. Indoor - fairly similar, generally need a bit of reach to enjoy it.
Oh, and you might not consider it climbing, but they both loved Clip n Climb a couple of weeks ago
Harnesses - yes, we Rock Empire Apache, as the Petzl one didn't used to go small enough. But the ones Clip n Climb had were a different (new, I assume) design with adjustable leg loops that was fine. Helmets, Grivel used to do a kids Salamander that mine liked as they were the same as the helmets the rescue team had at the time!