Any band beyond a certain age are derided as 'has-beens' by younger fans - it's all part of the process of enjoying music.
Ideally, as a teenager, your music of choice should simultaniously bind you to your peer group, and seriously get up the noses of your parents. This is even true now, when the current generation of teens have parents who grew up with pop and rock themselves, so it makes sense to diss the 'oldies' and their choices.
For the record, Quo have had more hits than any other British band since charts began, and they still have a massive world-wide fan base and tour regularly.
Best just to smile and nod - it's an argument you will never win.