It is true that bitches in season can not be exhibited, but unless you train in a bubble, and only attend obedience specialties, it is very likely that, at some point, your male will be 'thinking' with something other then his brain. Of course, there are ways to get around these problems. Neutering or spaying is the easiest solution. If this is not an option, it just means that you will need to be aware of these pit-falls, and train both you and your dog to cope with them. This decision is totally dependent on your own taste.
Age is irrelevant as far as ease of training.
But be warned, especially if this will be your first obedience dog, you may enjoy this so much that you will be hooked forever and want to continue in training. If you plan to do advance training with your dog, make sure that you give yourself enough time for training, and that your dog will still be physically able to accomplish your goals. Physical soundness is a major concern, especially if you plan to do advance training. Fiddle fronts, knuckling over, and weak rears are just some of the problems that can affect a dog's performance. Some people try to compensate fro an unsound animal and tend to keep their dogs on a light side weight wise to minimize the stress and trauma caused by jumping.