Pilates for flexibility?

Not sure how you'd measure. Not great around lower back and hips in my estimation, but not bad. Not prone to aches and pains off the bike. The difference for me is less about increased flexibility as the muscle strength to actually support and use that flexibility. All very well being able to get into a tuck, but if you seize up after 10 seconds (which I used to) and have to go back to the tops, it's not much good.
I'd say my range has increased on some flexes over time, but couldn't quantify it usefully.
Is that any help?

I'll admit to over 40. We have some errr... mature ... ladies in the class, who are improving week by week. So I suspect you can improve at any age. Though turnaround probably varies a lot.

First about 3 sessions made a huge difference. Came out pretty energised from the classes, and with core switched on, so it changed my posture riding home. Recreating that feeling every time I remember to on a ride has helped between classes. I cycle to work, and don't drive so that's at least 12 rides per week on average. Typical week is about 12-15 hours on the bike.

Since then got a pre ride upper body workout that I use before planned long rides to make sure core is working from start, and using some of the leg stretches post ride to counter muscle shortening.

So 6 months is quite a long time if you think I do 1 class and a dozen or more mini sessions per week. And I still do get some discomfort after long rides, though the time before getting uncomfortable is getting longer. Plus I have barely dipped in mileage over winter, I'm over 3000 miles this calendar year as of today.

1 thing to watch out for is cramp. I get foot cramp from the exercises quite often, which I think is related to riding. Bananas and milk before the class seem to offset that.

+1 for a class. I have a couple of DVDs and a real instructor who comes and prods you when you get it wrong makes all the difference. Plus it's a good laugh. Or at least the class I go to is.
From someone who could not get on the drops, I now ride on the drops mainly, I've dropped my stack height, and I get so flat on tri bars my knees are coming right up to my chest. Riding on the drops, I'm not putting my weight through my arms, so I have them bent, and loose, so no more ulna paralysis, and being able to drop my shoulders in, I don't get neck ache any more. All after 6 months of classes. 3 years of using the DVDs did nothing.
One other benefit, my musclature is rather different to the girls, none of whom cycle as much as I do, and we've had to tailor some of the exercises a little. Which my instructor has been brilliant at doing.