Getting an older dog is a good thing to do. They're already likely trained and won't have a 'puppy stage'. Plus, not many people want to adopt the poor things!
Puppies can be harder to take care of and need a LOT of time.
It depends on how much time you want to give to your dog. If you don't want a untrained and hyper puppy (Destructive things), then get one at least a year old (Two or so for Lab mixes). Other then that, it depends on if you mind that the dog is older. Some dogs can live up to 16+ years, and some only around 9 or so. Small dogs tend to live longer then big dogs, as well. Older dogs also need special food made for them (Often called the 'Senior' blend or something like that, it has extra stuff in it for bone health). Older dogs can also have problems depending on their breed, like Hip Dyspepsia or joint problems like Arthritis. They DO still need to be exorcised.
It's all up to you, but I'd actually say steer clear of puppies, as they can be more trouble than they are worth if you aren't strict with training and don't have a lot of time and energy to give to them!
Proud owner of two dogs, both destructive puppies! And my uncle adopts senior and non-puppy dogs, they are a lot calmer!