Most people I suppose will get a parrot when it is still a baby. What is a good age to sell them and what age are considered cut off ages for proper bonding?
This is an excellent question, and an area of some controversy right now. I'll stick to the main non-controversial points here, and will offer some personal experience. I won't try to settle the controversies, but hope that these reflections are useful for helping you in using your best judgement.
(1) Do not ever buy a baby parrot that is not weaned unless you know what you are doing when hand-feeding baby birds. (Hand-feeding poses the risk of aspiration, in which food can get in the baby's trachea and drown it.) Even then, wait until the baby has enough feathers to be able to regulate its own body temperature. This is not only to ensure that the baby can be handled safely--it also allows the parents sufficient time to feed the baby so that they can transfer antibodies to it and boost its immune system.
(2) Be aware of state and local regulations. In California, for example, it is prohibited to buy or sell and unweaned parrot.
(3) The received wisdom that an adult bird will not bond as well to its human as a baby has come under some criticism lately. I have adopted some adult birds from some unfortunate circumstances (one is on my shoulder right now), and have purchased some adult birds (one of whom loves to sleep leaned up against my cheek). I have found that, with patience and love, they have all become loving pets who have bonded with me.
When I raise birds, I start helping to hand feed them when their pinfeathers are well emerged, and I share the feeding responsibilities with the parents. (This technique, called shared socialization, requires some trust be built between the breeder and the parents, of course.) My birds have all gone on to bond with their new humans (the birds I could bring myself to give up, that is).
Parrots are highly intelligent, and their relationships with their parents might be more important than we currently understand. Since a weaned bird will happily bond with its humans, it may be prudent to leave the babies with their parents for a longer, rather than a shorter, amount of time and let them develop some confidence in their parents' care. A more grown baby will also have a more developed personality, and it will be easier to tell if the individual bird is a good match for the person involved. (How? The bird will tell you by it reactions to the humans.)