You will often hear the advice that joeys bond better than adults, but the truth is far more nuanced.
Joeys are a nearly blank slate. There will be some influence from their breeders, but most of their training and bonding will be up to the new owners. Bonding a joey is not particularly difficult, and a wonderfully heartwarming journey, but mistakes during this stage can have lasting consequences on their personality and development. New owners should prepare a good plan for training and bonding joeys while they are still young and malleable, even if they seem scared.
Young joeys tend to be a bit like puppies in their energy and enthusiasm. They will face hug, climb on your head, try to chew your fingertips and nails, attack your hair, stick their face in your ears, launch out of the cage during feedings, fishhook your eyelids with their nails as they scale your face, explore everything, nibble on most things, and are just generally more hyper and obnoxious than their adult selves will be. This is as fun and as exhausting as it sounds, but love it while it lasts because most gliders chill into their adult personalities between one and two years old. If they are still crazy past that, you’re probably stuck with it.
Joeys are the longest time commitment as well, living for roughly 13 years in captivity. This should be a consideration for teenagers who are likely to have major life changes over the next decade, because owning gliders may restrict the homes you can rent and the states you can move to for college or employment.
Adults that were well handled and bonded to their previous owners will often bond to their new owners very quickly. They already trust humans and know the routines of being handled. Their personalities are known and well-developed, so the new owner can make an immediate judgement on whether the gliders are a good fit for them. With a well-handled adult glider, all of the hard training work is already done. I would expect them to walk onto your hand if offered, take treats, and show trust. They may still crab and even nip, and some well bonded gliders are terrible crabbers, but they probably won’t lunge or bite to bleed if you put a hand in the pouch.
I tend to suggest well-handled adults for new and younger owners, as I think it is much easier to handle a glider that is already comfortable with it. As an analogy, I would suggest an already trained horse to someone that wants to learn how to ride. Also, as a matter of morbid practicality, adults are a shorter time commitment if it turns out that gliders just aren’t your thing or your life changes drastically when you move off to college.
Adults with Issues
Adults that have personality or behavioral issues may be easier to find in rehoming ads since more people will have given up on them and sought a new home. These gliders must first unlearn their negative associations and habits before much progress can be made toward bonding, or you must learn how to accept the things they cannot change. Retraining glider behavior takes patience and persistence, and is usually very challenging for a new owner. It is a good idea to meet adult gliders to see if you are comfortable with their behaviors before committing. Some behavioral issues like pouch protectiveness are pretty easy to deal with, but medical issues and other serious complications are better dealt with by an experienced owner.
Vet care can be an expensive consideration when taking in adults that have not been well-cared for. An immediate vet check for a fecal exam to spot parasites is a good plan. It is more important to get them onto a good diet than to maintain continuity with familiar foods, so feel free to switch their diet if needed. An unbalanced diet can contribute to several glider health issues including seizures, hind leg paralysis from calcium deficiency, and orange/brown discoloration of the fur around the face, shoulders, and hips (orangish color on the head, chest, and pouch are not related to this diet staining).