Its sad when people reach a certain point, either in weight gain or age, where they feel there is no point in attempting to return to the stature of their youth. Most think its impossible, many have just lost the fire. The truth is just starting creates its own motivation and the results can be quicker than most people imagine. 12 weeks is enough to show dramatic increases in strength and 3-6 months provides the kind of lean muscle mass that keeps people motivated to at least maintain it.
A recent study on the elderly (approximate age was 92 years old) proves just this. The intervention group participated in a 12 week multicomponent exercise program consisting of reps in the 8-10 rep range at 40-60% 1RM (one rep max). This is not a bad place to start for just about anyone considering neural adaptations to the movement are what provide the initial jump in strength from session to session for the first couple months. They performed this exercise program twice a week. Again, this is a great place to start, especially for those in their 90s!
The study found all markers of health and strength improved compared to their peers.
The intervention group showed significantly improved TUG with single and dual tasks, rise from a chair and balance performance (P < 0.01), and a reduced incidence of falls. In addition, the intervention group showed enhanced muscle power and strength (P < 0.01). Moreover, there were significant increases in the total and high-density muscle cross-sectional area in the intervention group. The control group significantly reduced strength and functional outcomes. Routine multicomponent exercise intervention should be prescribed to nonagenarians because overall physical outcomes are improved in this population
What would have been great is if they followed these same individuals for another 12 weeks in a periodized program where the weights were increased upwards of 80-90% 1RM and/or the volume was increased. This would almost certainly have provided further gains.
The point isn't really what, how or exactly what they did to obtain these gains. The simple fact that individuals of this advanced age can still make these gains without injury and actually reduce causes of injury for that age group, like falls, is reason alone for those who think its too late to get started.
Like I keep insisting on here, keep it simple. Stick to just your basic exercise movements; push-ups, chin-ups, overhead presses, rows, squats and deadlifts and the rest will fill in itself. Don't worry about all the fad exercise programs that insist you hit this and that over and over. These compound movements target multiple muscle groups at once. Beginners especially need to work on this before worrying about targeting individual muscles, if ever having to worry about that.
Read the study here if you want to dig into it further: