Overview of Temple of Avo Hero Save Age Reduction Trick -
Since I couldn't find one basic explanation -
There are two ways to reduce your age - monetary donations at the Temple of Avo and human donations to the Chapel of Skorm.
There is a way to maximize the benefits of age reduction by using the Hero Save.
During the Find/Rescue the Archaeologist Quest, you can go to the Temple and make a donation. If the donation is large enough, you receive an age reduction. If you then do a Hero Save, exit the game, and restart, you can do this again. Following is a somewhat atreamlined approach to maximize the benefits of this trick -
Start in Oakvale. Purchase the Marital Home (the house you grew up in as a child), and the three adjascent houses by killing the owners. Purchase the houses, and rent them out. Go to any one of the houses, break down the doors, and sleep in the bed 6-7 times to advance the game by three days. Leave the house, collect the rent, and repeat. I typically continue until I have over $100k in gold.
During this time, to reduce the cost requirement for age reduction at the Temple of Avo, go on a killing spree in Oakvale. Continue killing residents and gueards until your alignment is fully evil.
At this point, start the Find the Archaeologist Quest. Skip any quest-related activities, and go to the Temple. Donate $33k, which should be enough if you're fully evil. If not, continue donating until your age has been reduced.
Perform the Hero Save, and exit the Quest. Repeat the activities in Oakvale, and start the Quest again. You can repeat this process until you're 18 years old.
There are other ways to gain money quickly, but that is the one I prefer.