You cannot generalize like this.
With age you you develop wisdom. Your experiences make you better. But you are decaying and your strength, enthusiasm, biology, etc are all working against you. It depends a lot on the person. Everyone will be different.
You will again a lot from age. Everything you learn stacks. But it also decays. It's best to learn all you can when you are young because you have a very long time for it to work for you. There are things in youth that you have at your disposal that you simply won't have when you are old. It's not necessarily that you won't be able to do the same things when you get older but they will be much more difficult and so most people will give up before they achieve it and it will never be the same if you learn it in your youth.
You will find out when you get older. It is not something that can really be expressed or communicated. It is something felt. I wish I would have worked harder when I was younger because now it is more difficult but it is also easier in some ways. Harder to concentrate and remember but easier to understand. I think kids typically want things so bad they distract themselves from achiving them or they never try because they've been told they can't do it. A life time is a very very long time. You can't imagine how much you will have have done until you get to look back on it and see. It's like climbing a mountain. At the bottom all you see is something very difficult but when you are at the top you can look down and see how far you've climbed. Of course the air is thinner, you are exhausted, and very sore but you have a perspective that you cannot fathom when you are young. When you are young you can only look up. When you are old you can only look down. (not entirely true but almost) Again, it means nothing until it means something. Some young people have more wisdom for their age than average. It is not a fault of age so much that our society isn't good at climbing mountains. I do think that is changing to some degree due to the "information highway" and over time, as long as humanity isn't destroyed by psychopathy, I think the youth will realize how much they can achieve and they will start to pass that down to their children. This is already done to some degree by a few but it will accelerate both in speed and size exponentially. It's already turned the cusp.
E.g., to be more specific. Take memorization. Memorization isn't a single thing. It is many things and depends on many factors such as health, experience, desire, etc. Because there are many variables all working to create the overall effect you can have some factors getting worse while others get better and the overall effect depends on all of that. It might increase for some and degree for others. Being chronically sick or unhealthy can destroy your memory and/or body. Depression can also cause major issues with memorization and other abilities.