IMO, it's largely based around the following highly subjective calculation:

((value you will add to client's bottom line) + (your relevant experience & portfolio) + (speed and accuracy of delivery) + (kudos)) * (budget & urgency of project delivery) * (industry) = rate per hour

You can't really get to a figure from what i've said above, but i just wanted to illustrate a number of factors that can go into a rate that's attractive and workable to both parties.

Age doesn't matter, but (relevant) experience does.