I’m now 15 and have noticed my hair part widening. it is much wider than when i was younger. i’ve always had fine hair but very thick density which could be why the part is visible as my individual hairs are very fine.
One thing I do when I think I have a thin spot is take a lock of hair from that area, cut it, and look at the individual hairs to see if there are thin hairs mixed among the thick. You can try that.