I would say from age 3 or so. You could consider the Happi toothbrush, they’re very suitable for young kids, but a bit more expensive.
https://www.happiteeth.com/?gclid=EAIaIQobChMIzNaM54jo7wIVwgRyCh3xrQxUEAAYASAAEgJx-vD_BwE

My 4yo has just lost 2 bottom teeth we think we will get her an electric one when those adult ones come through

Manual tooth brush is better than electric one for kids but if u want i think 8 or 9 years old is fine

I think once they can use a manual correctly there is no difference, also I have heard that water flossers are good too.

An electric toothbrush cleans your teeth much better than a normal toothbrush. It is at a constant time pattern, no variation. Advice given to my by a Dentist. Ours started at about 3.5 y,o, No trouble getting them to clean their teeth. Kids get very independent by then and object to you helping

My daughter’s are 7 and 10 and have had electric toothbrushes for about 2 years now. I find they actually enjoy brushing their teeth now and I don’t have to nag them to do it! Plus the dentist says they have great healthy teeth and gums so that’s another win!

My daughter is 2.5 and loves hers. I think any age is fine if they are happy to use it