As you know, Elly loves telling her children that their younger years are the best ones that they'll ever know; her love of doing so is seconded, oddly enough, by most of the other adult characters in the strip. This sort of thing bothers me because, like a lot of people, I couldn't wait to grow up and get my own life. The last three years or so are the best times I ever remember having so I find Elly's envy of youth annoying. It's as if she was about to, as off_model said, finish off by mournfully bleating the following demotivational speech:
The next 12 years will be the best years of your life, honey, after that life isn't worth living, really take it from me, all too soon you will be married to someone you don't love, raising kids you can't stand, taking care of pets you don't want, you feel empty inside and then you start seeking solace in food but you start gaining weight and you start to feel bad about yourself so you eat more junk and soon you're caught in a vicious cycle and it's just hell, just hell, so go out there and enjoy these years while you can because it doesn't get any better, the only thing to look forward to is hearing about how others have it worse than you and I guess there's some comfort in that, but it's a bittersweet comfort that doesn't help you sleep at night.
The sad part, of course, is that since the children are surrounded by envious creeps trying very hard to cover up their feelings of inadequacy and self-loathing by saying they don't miss the opportunities in life they missed out on, not them, nosirree, they come to believe the lie. I think the only reason that Liz settled for Anthony and vice versa was the shared delusion that life actually couldn't get any better so they might as well be miserable with someone familiar.