I read once that Han was supposed to be 29 or 30 in A New Hope
, meaning if accurate that he would have been just nine, ten or eleven years old at the most when the Clone Wars ended and the Empire was born. If Rebels
is set, say, halfway between Episode III and the original film then that would place Han in his late teens and roughly the age when youth sign up for the Imperial Academy to begin careers.
I'd been hearing the estimate of about 29 even before then, so I'd say the 29-30 range would probably be accurate given Harrison Ford's age at the time the original movie was filmed (34).