How much muscle/strength could I really expect to gain over age 50? I'm lifting on a regular basis, eating healthy, taking in a lot of protein, have been tested for testosterone levels (at 480 which is avg/high for my age) - so, can I make great gains? no gains? or expect to lose as I age?
If you want to make gains like a 20 year old, you'll have to have the testosterone and estrogen levels of a 20 year old.
On the surface, this doesn't answer your question... but it does, please follow:
Your first question is, "How much muscle/strength could I really expect to gain over age 50?" I am not sure that this is answerable. In what way would you like to measure it?
The second question is, "can I make great gains?" This is also an opinion question, I believe you'll make great gains but will you believe that they're great gains?
The third question is, "no gains?" This one I can answer, YOU WILL MAKE GAINS. As @rrirower pointed out, gains will be slower than you remembered but they will happen. I know bodybuilders who NEVER competed or took the sport seriously until they were eligible for the masters class - and they did well.
The fourth question is, "expect to lose as I age?" If you maintain an active lifestyle, your body will be forced to maintain as well. You'll only "lose" when your body doesn't feel that it needs the muscle anymore (because you're not lifting) which it will then burn for energy, just as we burn fat and muscle for energy in a caloric deficit (diet).
Therefore, you will make gains. I hope you will be patient enough to accept them as great gains.
Sarcopenia. A five syllable word, a fancy way of saying 'use em or lose em'. Fact is, people who do not exercise past the age of 35 can expect to lose 1% or so of their body muscle mass on an annual basis. Weightlifting and jogging are not options. Weight lifting and jogging as you age will keep you out of wheel chairs and assisted living centers until the moment comes when you meet your maker.