In my 20's and early 30's I could hit it 6x/wk, no problem. I always took the day off after leg day.
Long layoffs are much harder to bounce back from. Taking 1 week off every few months is good for you physically and mentally and you will not lose muscle.
As you get older you need more rest. Very difficult for anyone over 45 (natty) to train more than 5 days/wk and make progress, even if your diet is meticulous.
Training hard and smart will enable you to stay strong as you age. However, strength and energy levels naturally decrease with age now matter what you do.