I don't think your age is relevant - more what suits your face shape. I do think that as you age your hair changes texture, and your face changes appearance too and sometimes long hair can look a bit severe, while a shorter cut can be softer and more flattering.

If you have healthy hair and it suits you long, who cares what your age is. I'm almost 41 and grew my hair (again) after DD2 was born 3 years ago because it was so much easier on those tough days to just throw it up in a pony tail. I was starting to feel a bit frumpy though and so today got it all cut off into a short gamine cut and damn it looks good! It made my hairdresser's day too when I said just take it all off!!