This is utter BS, I'm afraid. In the first place, not every human is the same. I hadn't even stopped growing when I was eighteen, let alone built all the muscle I subsequently acquired. Cycling is an aerobic sport. The cardiovascular system, and especially the capillaries supplying oxygen to the muscles, grows with training. Cyclists in their late twenties are better than they were at eighteen precisely because they don't maintain the same level of fitness, they get fitter and better adapted for cycling as they get older and stronger, and it is simply impossible to acquire that level of fitness and adaptation while still in your teens.

OP, I'm 55, nearly 56. I am certainly not as quick as I was when I was younger, and not just because I'm about a stone heavier. But if you are 25lbs overweight, blame the weight before you worry about your age.

Two things get more difficult as you get older - losing any extra weight you happen to acquire, and maintaining the same level of muscle mass. The latter takes more work than before, and ultimately becomes impossible, because testosterone levels drop with age. Hence the expression - someone in here has it as their signature - "I'm not as good as I once was, but I'm as good once as I ever was." The good news is that the engine stays in pretty good shape, and stamina is retained much better than speed. If you want to maintain your current level of performance as long as possible, don't increase your saddle time - if anything, you can probably afford to reduce it. Get into the gym and do some weight training: it'll help maintain the muscle mass that cycling needs, but isn't great at developing.