Dogs can be trained at any age.
Both of our boys (Deutsche Wusshund, and English Mastiff) were rescue dogs.
By nature, dogs are intelligent, and cling to a pack social order. They very much want to please the pack leader (you). If the owner asserts him/herself as the strong leader, the dogs will feel more secure & confident, making learning much easier. Through repetition, they will learn, usually rather quickly.

I taught our boys to respond not only to verbal commands, but also hand gestures. Comes in handy, when I am in situations where I do not wish to raise my voice, to command them.