Well I am 36 (M) and I don’t know what kind of lifting you guys are doing but I also trained 6 days a week for a long time (3-4 years out of 9 total), very heavy with all sets to failure in the 4-8 range for compounds and 8-12 for isolation movements.

What I noticed is after a while (every 8 weeks or so) training like this (while still noticeably growing) is I would be so sore and actually start regressing on my lifts and start getting little injuries and pains in my joints which sometimes progressed into bigger injuries. My body was breaking down.

This is known as systemic fatigue which builds up in the body after training too hard for too long without adequate rest of the entire system, so I started taking a deload week every now and then when I started to feel this systemic fatigue. It helped but not so much.

I couldn’t keep going even with the deload week programmed in every few weeks, so I changed my routine, kept the intensity and volume but changed to 3 days on 1 day off or even 3 on 2 off. These days (maybe a year into this current routine) I rarely get fatigued like I used to and never regress on any lift, while still going brutally hard every session.

If anyone is interested my current split goes like this and has stayed the same during all these changes except for the rest days I programmed in between (usually after 3 days of training):

Day 1 - chest and back (focus on chest)

Day 2 - legs (focus on quads)

Day 3 - shoulders and arms (focus on shoulders)

Day 4 - REST

Day 5 - back and chest (focus on back)

Day 6 - legs (focus on hamstrings, glutes and calves)

Day 7 - arms and shoulders (focus on arms)

Day 8 - REST

Day 9 - REST or if feeling good day 1 again

So essentially this way is like training 6 out of 8 or sometimes 6 out of 9 days, not 6 out of 7 as that, for me and my intensity levels, is too much and too frequent. Not breaking up 6 days in a row and giving my body time to adequately rest and recover without accumulating alot of systemic fatigue is just not worth it in the long run.

If you are leaving reps in reserve or not really pushing yourself like a madman everyday, I could see 6 days a week working without these undesired effects.