Well, I'm a relatively unfit 63 yo. I walk a lot but have a sedentary job and do very little with my upper body. I did the YouTube beginner epic starting in the first week of January and was very sore after the first three days. I could hardly put my hat on because of the triceps DOMS! But finished it off and now repeating. It's definitely better this time around, not much soreness at all though it may help that I've had time this week to start doing some stretchy yoga in the evening. Today I did day 4 with weights. Obviously much smaller than hers but I didn't need the 5lbs at all , used the 3kgs for most and 10 for quite a few. I could have done with a bit more for the RDLs but DH is away and took one of the 6kg pair.
So... yes I found it a bit brutal (and there's a few things I can't do, eg I have to modify most side planks) but it didn't kill me and I think has already made me that bit stronger.