As far as casual battles with friends, Lv X cards can still be quite good. Older cards aren't necessarily worse than new cards (shout out to the MVP of my Grass deck, Slice Drain Sceptile), so if there's some good Lv X's in your collection, there's no reason you can't use them! I don't know how the online TCG works, but if there's no age limit on cards then it's the same answer :)