Help you do what, exactly? This reeks a lot of being a homework assignment; if it is, admit it up front.

What have you tried? It's pretty trivial, in either Perl or JavaScript to write a regex (that would look something like this: m/^[0-9]{1,3}$/ to check for 1 through 3 digits in a field. In JavaScript, you could also do a string->integer conversion (there's a function) and handle the error if /^\D/ and the range if 0 > entry > 999 (do you really want to allow ages to 999? I don't know; you didn't specify it!