There is no official age restriction at all. The oldest astronaut with the US space program so far has been 46; the average age is 34.
However, you do have to pass a number of very strict tests of physical and
mental fitness incuding eyesight etc, so a much older person would have to be
in very good health to qualify. There is, however, no real reason why this
shouldn't happen. As for a lower age limit, again there isn't one officially.
But as you have to have a college education and a minimum of three years work
experience related to the field - as a pilot, for example - it doesn't seem
likely that a very young person would qualify. 26 seems to be about the
youngest so far.

On this subject: the NASA website specifically states that they have no plans to send children into space at this time!