Is there an age range at which adding muscle mass becomes more or less a losing battle? That is, no matter how much one lifts or how often one works out, building muscle just ceases?
I hope I like the answers you get. I am 65 and have been noticing some strange shifting of the muscle mass.
Go to youtube and search by ‘fitness in old age’ and you’ll see videos of people in their 60s, 70s and 90s who are fit. Awhile back I saw one with a woman who was being filmed at a gym by a local TV station program. I don’t recall the weight she was pressing, but she did 20 leg presses. The TV newswoman got the beefiest looking guys to give it a shot. The one who came closest only could do 13 of the weight the woman pressed.
I’m 63, btw and a couple months ago I read how just doing a lot of pushups was good for your overall strength. So I gave it a shot. Now, mind you the numbers I got were of me taking it easy and just doing a few at a time for most of a day, on 3 separate days. I did 600, then 700, and a week later 800 in a day. No Jack Lalanne but pretty good for an unknown old guy, huh?
Go online and check out Jim Morris, a ripped and muscular bodybuilder who’s currently 77 years old. Here’s an interview to get you started.
There’s also this guy, but he owes much of his physique to HGH and a questionable program known as Cenegenics. I believe he’s since split from the Cenegenics program and gone out on his own with a program which is less controversial, even though he still uses his Cenegenics-created physique to sell his own program.
Additionally, I’m almost 65 and have been doing a lot of bicycling the last couple of years, and my leg muscles have definitely been growing and firming up.
So yes, muscle development is indeed possible as one gets older.
ETA: YM(literally!)MV.
Extraterrestrials don’t count. 
Seriously, the guy wasn’t human. But he did a hell of a lot for the fitness industry.
Clarence Bass was in his 40’s when he started bodybuilding and is in his mid-late 70’s and is still doing it.
He’s written several books and used to do a column in muscle and Fitness called Ripped
Older adults may not build muscle mass as fast as a 20 year old, but they certainly can maintain or add muscle. They have to do some work, though, there aren’t any short cuts.
It defiantly does not go as fast as it goes with the youngsters and I’m not even 60 yet.
It takes a lot of care about what you eat, how you train and avoiding injuries (since when you’re older you are more likely to pick one up).
However, it sure is possible to get ripped – but it requires training, training and some training.
One thing is certain - without regular resistance exercise and decent nutrition you will lose muscle mass and strength as you age. The official term for that is “sarcopenia” and if you are inactive that muscle loss can begin as early as 30, 3 to 5% of your muscle mass per decade. That loss speeds up some time after 45 to 1% per year and really gets going sometime after 65.
So you are having to swim against the current, so to speak. Do nothing and you lose it. Do nothing and you will be a frail person before you know it and even if your BMI is okay it will be mostly fat and paunch. Work moderately hard to stay in the same place. Making progress is possibe but it takes more effort. Very doable.
In my 20s and 30s I lifted weights, jogged, did squats, deadlifts, probably 3x/week for years. I worked up to doing parallel squats with >385lbs for 4-5 reps. I had strong legs, for sure, but the hypertrophy and strength I developed was not nearly as dramatic as when I retired and started riding a bicycle.
When I was 53, I started riding a bike daily (the day I retired). Since I was not a lightweight, riding hills was not easy. Still I kept it up not missing a day for a full year.
At the end of the second year, my legs, hamstrings, quadriceps were much larger than they ever got from lifting regular barbells.
I attribute this to two things. One, after being retired, I never missed a workout. Two, riding a bike was not so demanding that I couldn’t recover in a relatively short period, such that I could sometimes ride twice a day.
Thus, in my youth, I’d workout about 12x/month, and maybe miss one or two days (due in part to working swing shifts). But in my retirement I was ‘working out’ 7 days a week, or 30 times per month. Measured over a year, that’s 120-140 weight lifting workouts versus 360 bicycling workouts.
So, good genetics, not getting injured, consistency and frequency meant that I actually gained more muscle in my 50s and beyond than in my 30s, though the effort per workout seemed greater and more impressive doing heavy squats.
I just couldn’t equal the consistency and frequency I got in my retirement years when I was younger and stronger and possibly more motivated, due to having an 8 hr/day job.
So it’s not ‘more effort’, it’s more consistency, more frequent sessions (up to twice a day during the Summer) that lead to building more muscle later in life than even in youth. Of course you can’t over train, you can’t get injured and you have to have decent nutrition and decent genetics.