Its all depend on the dog that you adopting, if you adopt a dog having calm temperament then you are right away go for it, but if in case if you are adopting the dog having "Marley" temperament (watch Marley and Me hollywood movie, you come to know it) then you will be in trouble. First you need to figure out that the dog you are adopting belongs to field line(american) labrador or british (english) labrador. Basically british labrador are easy to calm down than the field labrador. You need to do lot of research before adopting adult labrador, Even I too had this idea in my mind i.e. adopting adult lab but later i got puppy as i felt my dog care experience will be only fulfilled if i get a dog from his childhood onwards.
If you are strong enough to handle such a big dog while walking or playing then go for it, else better go for puppy itself.