whats the desired frequency/intensity to achieve these results optimally? And say if it's 3/week @~60%+ , is there really such significant difference between once , twice, and a third time? Obviously there will be diminishing returns, just wondering if you have an idea of how great that may be

untrained, early 50's, not overweight, no health issues to note

boom 100 %

Honestly at that age as long as theyre active and moving. I help out a few seniors in their late 50s and early 60s. I have them go 4-5 times a week depending on their other commitments. They eat 5 clean meals a day and take proper supplements. Really depends how far you want to go and your willpower.. Hope this helps!

I believe the research suggests a minimum of 1 to 2 weightlifting sessions per week to maintain strength in older or average individuals. I personally believe that increases for anyone looking to be competitive or above average. Set your weekly minimum at 2x per week if you're getting them to do a full body routine but make it count, use good complex movements. Don't pussy foot around with bicep curls, tricep extensions and crunches. Optimally I would suggest they workout 4 times per week if not 5 (wean into 5x with lower volume first, they still have limitations with aging) with aerobic work on non-resistance days.

The active and moving mentality doesnt work well because everybody has different expectations and perceptions of what activity is. It's better to set absolute, objective standards and have people attempt to meet them as well as they can. If you tell me to stay active and moving I'll be in the gym squatting an easy 135 for 30 reps every single day. You tell the same thing to an old person and they make an effort to get off their couch and swing their arms around a bit more every day. Granted older people have greater limitations than I do but realistically that should only reduce the weight they are squatting, not restrict them (in most cases) from squatting.

thats why i said based on how far they want to go. they tell me their goal and i give them a plan!

What do you mean how far they want to go? Are you talking physical distances or body comp goals? They shouldn't be given the option of complacency. Fitness is a life long battle of constant improvement. There should be no stopping if you have room for improvement (obviously your cap decreases slowly with age)

youre talking to a competitive bodybuilder. So yes theres a differnece how "far they want to go" does gramps wanna look like stalone? or just be healthy.

To be honest (without putting someone in juice) looking like stallone and being healthy are very similar goals.

Lol youre talking one extreme to something way more attainable with just simple consistency.