This is a rather complex issue.
Just my opinion.
If you are going to compete in “hypothetical” clean amateur races. Then ‘hypothetically’ you cannot use testosterone. However, many do, as there’s next to zero drug testing. Really, it’s kinda irrelevant as there’s normally always stronger riders. Usually, just because they are younger / better etc.
So, in the scheme of things, it’s just an older racer, who happens to have the ‘ability’ of a younger racer. In a mass start event. I guess it’s irrelevant.
For age category racing, it is absolutely cheating, unless of course a user just informed everyone they were racing that they were actually a young person 
Not likely.
The interesting aspect is the sheer volume of cyclists that cheat, when the barrier is low and the penalty is minor.
Zwift has taught us this. I once thought it was a small percentage of humans that would cheat to win. How amusing, in retrospect.
In online racing the percentage of cheaters is crazy. If that was the percentage across all humans on Earth, we’d be more doomed than we already are.