I think it's been a progressive thing, definitely slowing after 43 and continuing. I am 53 and have run for 38 years. I spent much of my 30s being pregnant or getting back in shape, although I did run pretty much every day for many of those years. I'd work my way back into racing shape and then get pregnant again, so those years were kind of "off", although I'd get back to 60 mpw. For many years, I would average 70 miles a week and even as a master, I routinely ran mid to high 60s all year, sometimes going up past 75 mpw.
For comparison sake, I ran a 1:19:01 half as a 24 year old, a 1:24:18 half as a 43 year old and ran a 1:29 half (very hilly) at 47, 1:30 half at 48 and a 1:32:46 half at 52. I don't know that I was in the same amount of fitness in each race, but you can see the slow down. I usually only race xc, and it's hard to compare courses, but it seems like the slow down is about the same.
It seems like the decline comes in spurts - age 44, 49, 52, etc.
I have generally run a lot of miles (since becoming a master) on trails/grass, so that helps, but I am noticing an increase in the number of injuries. Most are related to the amount of sitting and driving I do and my lack of strength in my hips/pelvis and the fact that I have a bunion on one foot that creates an even foot strike.
I had a very good year last fall, running better than I have in several years. This was despite being injured for several months (Dec-Feb). I attribute this to losing weight and getting back down to near what I weighed in my 20s. At the lighter weight, it was easier to fun fast, despite not training harder.
I'm currently battling some high hamstring tendonitis pain - again most likely related to sitting/driving, but also not being able to afford new shoes for a bit too long. I'm trying to be confident that I'll be in good shape by fall xc again, but I think a lot of that will depend on maintaining the lower weight and gaining strength and flexibility in my hips (also losing my very tight calves).
As we get older, we lose flexibility and strength and I've been really bad about weight lifting (which I did quite regularly in my 20s and 40s) and kind of bad about stretching (my schedule is such that I feel like i have to launch into running in order to make the most of my time - also, I tend to run for as long as I have time and would rather run another couple miles than stretch). My core is pretty good, but obviously I still have to keep working at that.
I also think the decline is somewhat individualized. There is a big decline after 50 for many people - and the injuries certainly start mounting up then. Many a master runner has been focused on kicking butt in the 50s, only to find out that they can't because they lost speed or get hurt. As a very fast friend put it recently. "50 sucks. Big time." ;-)