Hi and welcome. Kudos for wanting to ask questions before going out and snatching up the first boxer you see..Sadly a lot of people do that. Know what you are getting into. Boxers are energetic, fun loving clowns. They are also very social and like to be around their "peeps"..They are intelligent and just a bit hardheaded as well. But if you were to ask anyone on here, I don't think any of them regrets for one second the choice they made..

Do your research, talk to boxer people, if you can, visit shelters, breeders you name it. The more you know what to expect, the better off you will be...
Puppy vs Adult...Well now we all love those cute little coneheads, but a puppy isn't for everyone..I thought I had done tons of research before bringing home Buck, thought I was well prepared, knew it all....Boy was I kiddinig myself! 8 yrs later and I learn everyday...Like a lot of peole on here, we have gotten our boxers thru rescue and I really recomend it. Seldom do they have puppies, you generally won't know much about the background, but they are mostly housebroken and some have basic commands mastered..I've rescued 2, including my lovely senior Samson at the age of 6 and believe me, they are the most loving friends....
As far as well behaved, well that will totally up to you! It takes time, patience and a lot of work to achieve that, but very worth it..I recomend all dogs going thru a basic obedience course and actually going on to a secondary one as well...Work with your boxer daily and you will see the rewards. Just read in the training section some of the triumphs
Here's a post that I did in regards to finding a reputable breeder. Yes, you may pay more, but isn't it worth it knowing that your pup comes from parents that have had all necessary tests? Give that little sweetie the best start you can....And ask questions! If you don't like what you see, don't buy!
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