Different children, different rates of development.
DS1 (August) and DD1 (January)could walk at 11months, DS2(June) didn't walk till 14 or 15 months.
DS1 started school at 4 and a half,was always a little above average and got 2 good 'A' levels and 'U' on the third.
DD1 started school part-time at 3 and a half,was always well above average, has been considered G&T since Year 4 and is expected great things in her GCSE's this summer.
DS2 could read (pretty much self taught) at age 3,starting on level 4 books in Foundation, and is now well above average and predicted to achieve well at GCSE. His biggest problem is his awful,really difficult to read, handwriting.
I have never worried about their ages, because they are individuals. I have also never felt the need to have them tutored or anything extra.
I also recognise I have been very,very lucky that my children have all been high ability. As a teaching assistant, I know how very wide the variation in children's abilities and maturity can be, but as others have pointed out, quite often the oldest in the class is not the most able.