I think it was Friel who said that the most important thing is to never take extended breaks (several months? More? I can't remember) after a 'certain' age (35? 40? Again, I can't remember
). He wrote in his book that some research suggests that the best case scenario for performance decline while aging is a drop-off of about 2% per decade, but only in those who never took extended layoffs.
His version of the book I have is getting old, so I dunno if the research has been updated.
I'm in my mid-forties (ish), and some of my observations about myself are:
1) It takes much longer to warm up.
2) It takes me a bit longer to recover. If I was being honest with myself, I can probably only properly handle 2 big/hard days a week, or 5 a fortnight, at the most, rather than 3-a-week.
3) I have many less good days on the bike than I used to; I'd be lucky to have one "really good legs day" a week. This might be due my lack of taking notice of my "observation #1".
In my 20s, I could pretty much guarantee myself that, after a 10 min warm-up, I could TT like Cancellara, 3 times a week.
Well, at least that's what it felt like.
4) body weight is HEAPS harder to maintain. It seems as though I can only eat about 2/3 of what I could when I was in my late 20s/early 30s, but it's probably more like 3/4 to 80%.