Hmm. Going by Neville in the books, most magical children seem to have shown *some* signs of magic by the age of around eight, so anything from toddlerhood to there seems reasonable.
As for signs? Levitating something, summoning an object, breaking something they weren't touching - those sorts of things are always safe bets. I'd personally avoid stuff like levitating onto a roof or growing hair several inches overnight (Harry- level underage magic!) because that's likely to freak the muggle parent out I suspect!
Hope that helps a bit
~Carys