Autism - when can you stop worrying about it?

both my neice an nephew are autistic. my nephew is 5 and was diagnosed less than a year ago but his parents had suspisions that there was something wrong from an early age. his is on the more severe scale. his sister is 6 and was diagnosed around the same time as her brother and is on the lower end of the scale affecting her behavour. she can be very aggressive. i think if your child seems normal is developing normally then i wouldn't worry too much.