Bodybuilding is one of the few sports in which men can continue to be "competitive" well past their athletic prime. Starting at 38-39, while not ideal, still leaves plenty of time during which to add an incredible amount of muscle size. If the individual is drug-free, sure, they aren't going to get as far as they would have if they had started younger, but for the guys who use PEDs, a tremendous amount of muscle can be added within just 3-5 years of training...a tremendous amount.
At 40-45 years old (I am generalizing here) the body can still perform very well (from a muscle growth standpoint). However, when it comes to wear and tear, a man that age is going to have a lot more potential joint and connective tissue issues compared to a younger guy, as well as decreased recovery ability. While these things can hinder one's bodybuilding progress, it isn't going to make a giant difference...assuming the individual can still perform basic exercises. In many cases this negative can actually be turned into a positive, as it forces the individual to lift properly, rather than engaging in ego-lifting, which happens to the majority of younger lifters at one point or another.
The one downside to starting late is that the individual doesn't have as much time to enjoy the fruits of his labor. Unfortunately, if someone starts at around 40 and builds a competition worthy physique by age 45, they only have a few years left--maybe a decade--during which to maintain it. Once we start approaching 60 the body begins to decline no matter what we do. This is why we don't see any great bodybuilders (compared to younger bodybuilders) at age 60+. However, assuming the individual takes the right steps so that his body is still able to perform in older age, he can continue looking damn good all the way up into his 60's and possibly even his 70's. It is my opinion that just being able to maintain a "bodybuilding" physique into one's 60's is a great accomplishment...and one rarely encountered. Whenever I see a guy in his 60's who still looks like a bodybuilder (lean and carrying an age-appropriate level of muscle size) I am truly impressed. My hat goes off to such individuals. One of the biggest reasons why we don't see this more often is because few men can maintain the motivation and level of commitment necessary to continue looking like a bodybuilder into their 60's, let alone the physical ability to do so. Consequently, most 60+ year old men don't look that great, but the kind of physical deterioration typically witnessed in one's 60's is not inevitable. The body CAN maintain an impressive amount of muscle size and definition for as long as one is able to eat and train. No, you're skin's not going to be as tight as it was when you were younger, your midsection won't be as small, and your aesthetics will begin to fade, but a man can still look great at that age--better than most 20 year olds--aside from the unavoidable age related issues mentioned above.
So, if you are a 40 something year old man who regrets not starting earlier, rather than sitting around thinking about it, you should instead think about the accomplishments you can achieve in the decades to come. I promise you this--you will turn far more heards (and get way more compliments) if you can maintain a bodybuilder-like physique into you 60's. Even if you carry only 10-20 pounds more muscle than an average young man in his untrained state, while sitting at sub-10% BF, you will blow people's minds because so few men ever accomplish something like this.
