Lighting – Why do the fluorescent lights flicker and require cycling the switch to turn on

Bathroom has a 2 T-Fluorescent bulb fixture. The bulbs went out a 2-3 years ago I believe for the first time in 10+ years (when the fixture was installed). From the moment the bulb were installed, the lighting seemed peculiar. They have this extremely quick strobing effect so the light is not a solidly emitted. Fast forward to about a month ago and the bulbs had trouble turning on and the bulbs would stay dim at the ends or flicker a bit signifying that there is at least an electrical signal. One of the bulbs also has one black end if that means anything. Would need to flip the switch several times before the bulbs would fully light up, however once on they wouldn't go out.

So I did some research and I thought maybe the fixture and ballast is old and needs replacement. My dad says I just need to change the bulbs, but seeing as though I changed them quite recently (these bulbs are supposed to last tens of thousands of hours and we use them maybe an hour a day at most?), I didn't think that was likely. I thought at first perhaps I used t12 instead of t8 but just confirmed that the ballast states t12.

So now the bulbs won't light up unless we flick the switch several times. I don't really know anything about lighting at all, but I think the key fact is that the bulbs are STILL WORKING, and they both had problems at the same time, so I have to assume the bulbs are not dead, which is the most confusing part. Do fluorescents tend to malfunction rather than straight out die at the end of their life?

Can someone with expertise give their best opinion as to what's going on? Do these low-medium end fixtures tend to wear out after 15 years?

Thanks for your answers in advance.