Rules as Written don't appear to say. Agree with @agentwiggles during career mode. In game time, keeping track of 4 year intervals is hassle.
What seems a fair interpretation to me is that once you have learned a skill between 34 and 68 (twice 34), then you don't lose it but cognitive decline makes it harder to learn new skills.
After age of 68, then it is both harder to learn new skills and maintain your set of current skills.
The threshold changes if the PC lives on a TL 0 to TL 2 world, where quality of life is much harsher. Then new skill decline = current skill decline = age 34 and above.