It depends. Old dogs are great for low energy homes (I.e, if you aren't home a lot, don't have time to walk it all the time, live with older people, or have small children who've never had a dog before), but they're not that great if you want a dog you can do stuff with.

I had an older boxer growing up, and he was great. When we got him, he was on the cusp of being an old man. He still had some play in him, but after about half an hour, he was done for the day. Not a lot of fun to play with, but he was super great for just hanging out, and his low energy made him great with kids (He was basically a therapy dog for temper tantrums).

But having gotten older, and having raised my current boxer since she was a pup (9 weeks old), I can say it's a lot more fun to get them young. You get to watch them grow up, you can actually train them well, and they seem to bind with you a lot better(All my rescues have been very aloof, while all my pups have been much more affectionate and attached). Plus I can actually take the younger dogs out and about town without worrying if they're going to be exhausted after 20 minutes .