Teeth! What age is it normal to start losing milk teeth?

[chocolateshoes]

My 4 year old niece has just lost her 1st tooth & her parents are worried that this seems rather early? What do you think? My DS is only 7 months so I have no idea! Do they need to ring dentist?

ds1 lost his 1st tooth when he was 3 or 4. ds2 has yet to lose any, he is 5. ds1 is fine so far (7 on tuesday)