Xjyuk

I turned 37 this summer, and started ridding quite a lot (about 3500km in a little less than a year). When buying my carbon bike, and then aluminium commuter, I was careful to set the saddle height using the same method: putting the heel on the pedal and stretching the leg. That way, my pedal stroke has my legs still being slightly bent when reaching the furthest development. I had to fine-tune during the first week, but soon found the most comfortable position.

About 2 months ago, I started having the feeling that my knees bend a bit more than they use to. I also noticed sometimes light pain in the knee, the same as described when position is not good. It's not permanent, but it happens sometimes.

Oh, and my seat post hasn't gone down the tube. Saddle posts are marked, and didn't move.

Is it possible for my morphology to change with constant effort? I try to ride quite often 150km on the weekend and I commute everyday.

Your observations are confounded with time and lots of things can change in the intervening time, this include your flexibility (e.g., hips and/or hamstrings) and your components (e.g., your saddle).

Flexibility, is often the first thing to change, if you were relatively inflexible in your hips you may have adopted posterior rotation of your hips (i.e., the top of your hips backwards) when riding, which could effectively shorten your leg length on the saddle. This is often part and parcel with short hamstrings. As flexibility improves you may have been able to rotate your pelvis to a more neutral position, which would have effectively lengthened your leg reach. As an aside, flexibility is also one of the first places people should look when they think they have a leg length discrepancy, structural leg length discrepancies are much rarer than asymmetrical flexibility.

Equipment can also change over time. If you are running one of the classic leather hammock style saddles (e.g., Brooks) these can stretch and sag over time, which would lower the effective saddle height. Even some modern synthetic saddles can "break-in" (i.e., the supporting plastic frame becomes more compliant), which would cause the top of the saddle to sag a bit more than new when you put your full weight on it. This might be harder to pick up as the saddle should return to the original height when you get off, due to the memory property of plastics.

Sorry, you are not growing

It is highly improbable that you are growing at 37, as ~19-21 is the maximum age by which most people stop growing. It is possible to develop a tumor that stimulates your pituitary to produce excess growth hormone, this actually happened to a friend, but since you have stopped growing in height this will result in your bones getting thicker and heavier not longer. Gigantism can also cause excessive height, but would have shown up earlier in life during your growth phase.

I have a similar result, and it came from increased development in the calf muscles due to riding on a recumbent and holding a relatively fixed ankle angle.

So when I jump on my road bike I can hold a pointier toe and that's effectively making my leg longer.

Your only option is to raise the saddle by a tiny amount at a time, until your hips start rocking while pedalling, then drop it back down till they stay horizontal.

Take this as a visible indication of how much your muscles have improved.

This can also be a general improvement in your flexibility due to exercise. Keep it up!