Nothing wrong with training a 9 month old. I think it has some benefits. Already toilet trained, The puppy energy will be leaving and it will be wanting to put in some serious work if its a good dog. As long as the puppy has good ground work it shouldn't matter. Good socialisation with people and dogs, not just playing with dogs and people but having manners and not rushing up to every dog to say hello etc.
There might be a few smaller things you will have to start from the start but if you can handle dogs no biggie.
Got a 6 year old dog as a first dog. Taught her to find my brass, take off shoes and socks and all sorts of cool stuff, bring the mail in. Just couldn't get her to stop chasing cats.
2nd dog was 9 months old and had spent 3 months in spca and foster.Got on well with most dogs and didn't try pull you over when walking past other dogs.
Had basically shat on concrete 1/2 her life. So toilet training her was hard as she missed the early learning stage. But the biggest thing I noticed was while walking on leash she would get caught on every post and pole possible. Older dog always walked your side of the post. Its the simple things haha.
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