Agree it totally depends on the individual child. I teach a lot of very small dancers, they're supposed to be 3 to start but some absolutely can't wait another minute at 2.9 and 2.10, and they absolutely love it. Others really can't cope with this sort of stuff until more like 5yo. Also depends on how 'formal' the activity is, in the case of my dance class it's purely for fun and tbh a bit bonkers
Do you think there is a time limit on how long kids can keep their attention? - yes, and it's the teachers responsibility to ensure that the session length and content is suitable. ime 3 yo's concentrate for 30-40 minutes, with breaks about every 10-15 mins and suitable variation of activity.
Mine all started ballet at 2-2.6yo. But they've an absolutely fantastic teacher who's brilliant with the younger ones and knows how to hold their attention. (30 minutes) Dd1 started swimming at 3.6yo which was fine for her as she's got good attention, but some of the other children had problems holding attention for that length of time. In retrospect she would have been better leaving until 4yo and I don't think it'd have made any difference to when she learnt to swim. (30 minutes) Dd2 started gym at 3.10yo and loved it but ds tried it at 3.4yo and didn't have the concentration or listening ability to be worthwhile. He stopped it after 1 term. (40 minutes) All except the swimming did taster sessions.