My four year old yellow bellied slider has some strange lines in his plastron. I couldn't tell how long have those been there, I think for a long time, maybe even years, but it's something that's starting to worry me a bit.
The plastron itself feels hard and firm, carefully pressing it doesn't give in at all, even with a moderate pressure.
Other than this he looks (and behaves) healthily.
You can see the straight lines that cross the lower scutes from side to side. They are clearer yellow than the rest of the plastron, with a pinkish halo around them.
I've emphasised the colours in this picture to make the lines more apparent.