I suppose it all depends on the tiel's history, personality, and previous training.
A handfed baby has it's advantages basically because you know what you're getting. They're accustomed to handling, and very easy to train. They also adapt easily to change, and won't be as stressed out as an older cockatiel may be in a new home.
You also know the birds history---and if you buy from a breeder, you can always keep in contact with them to phone in case you have troubles, or need to give the bird a home (for any unforseen occurances in the future.)
An older cockatiel may not be as tame, as their previous owners may not have handled it well. They may also not be as healthy as they could be--considering again, that previous owners may not have cared much about their birds diet. You never know.
They're also much more difficult to tame, than young birds, and may have bad habits and personality traits that resulted from their previous care.
With any amount of work though, your bird will bond too you. Older tiels are just like humans--more set in their ways. So it will take longer for the bird to adapt. But that's not saying it won't love you just as much as a young tiel.