Why do young people look better than old?

As stated, your question implies facts that are not necessarily valid.
While there certainly are some younger people who look better than some older people, there are also some really hideous looking younger people.
Much of appearance is nothing more than what you have done to yourself in a deluded belief that your body needs decoration.
And for that matter, beauty and attraction are often more subjective and not any kind of real evaluation.
But the real question becomes, what is the basis of beauty?
Some suggest that beauty is nothing more than sexual attraction. If that is the case then beauty would be an individual's way of determining who may be more likely to be a successful parent to your future children.
A wealthy and powerful man may be perceived as a guarantee that a woman's offspring will have their future needs met, while another woman may perceive a healthy and strong man as more capable of providing for the needs of her future children. In both cases, the attraction may be something that the woman cannot quantify as to why they believe that the man is attractive.
Some men prefer blondes, some prefer brunettes, and some don't care. Some prefer a slim figure, some prefer a few more curves and again, some don't care. In the case of a man determining the relative attractiveness of a female, it's often more of an appearance of good general health... but in general, younger females tend to be more capable of providing future children.
But these tendencies are part of the history of our species. It's what we have done for countless generations.
You will probably never be able to make a conscious decision to reverse what you perceive as better looking, it's just part of being the human animal.