I'll answer by the same number order you gave; by the way I'm 51.
1) I still ride my steel road race bike I use to race on 20 years ago! With tight racing geometry.
2) I do not take any special suppliments, in fact take less now then I did 20 years ago because I learned most of it is rip off.
3) Yes, I have slowed down; but due to time constraints with family and work commitments, I no longer ride 1,000 miles a month like I did in my race days, now I average 400 to 450 miles a month. So most of my slowing is due to training and a small percentage of that is due to age.
4) I've been riding bikes all my life; I started riding seriously when I was 17 but then stopped to buy a car and do the girl scene, then got back into again competitively at age 19 and raced until age 35. All during that time I also used the bike to commute in addition to my training miles.
5) I ride a rode bike exclusively, did dabble with MTB for about a year back in 87 but didn't like it. Most of my riding is what I call short to medium distance-3 days out of the week I'm doing 18-30 miles each day and one day out of the week I'll do 60-100 miles; and I still commute. Up intil 6 months ago I lived in California (since 15) and most of my riding there was in the mountains even up intill the time I left, now I live in Indiana and it's mostly flat here.
6) My monthly mileage average is between 400 and 450 miles.
7) I weight train about 3 days a week (when I'm not on the bike), but don't push a huge amount of weight to look like the Hulk-my body type wouldn't look like that anyway!!