I don't know about specific age, but I think the best time to do something like dying hair is when you can make a mature and concise decision.
So maybe when you're a bit older. The usual consensus on best dye colour for long hair is usally the primary hair colours e.g. ginger, brown, blond and black... but then again, best colour is a very subjective opinion, so go with what you think is best.
Personally if I was getting my hair dyed I would want rainbow colour just to be like one of the best characters from mlp: rainbow dash.