Of course, you're not at all too old at 21.
Most people are in university till the age of 22 or so anyway.
Why would 21 be too late?
I entered university at 17 and was there for five years.
Went back at 37 and earned the qualifications that have given me my career since.
If you have the slightest yen to go to university and can make it happen,
do it, I say.