Even as a kid in the 80's I though TOS was pretty dated. I've come to appreciate it as I've got older and there are certainly some great episodes but I still have that feeling.
I think a distinction needs to be made between aged and dating. TOS has aged and it looks like a series filmed in the 60s. But I don't think it looks dated. Then again I grew up in the 60s and 70s.
However, I feel the first 2 seasons of TNG look dated but the rest of the series still looks fine.