Speaking personally, osu has been pretty hard on my eyes (but as many people pointed out, may just be from looking at monitors for extended periods of time). Age also has an impact on me i'm sure. One issue I have with osu in general is after playing for ~1 hour, my eyes get blurry, if I keep playing it gets to the point of not being able to read words on web pages without zooming in. So for me, the possibility of osu being bad for my eyes is pretty high. Just take breaks and you should be fine.