It is not about the age, it is about the time you work in a specific field.

If you are newcomer to a field you simply don't know what is difficult and what is not. So you have a more open mind.

See for example https://en.wikipedia.org/wiki/Yuval_Ne'eman

This guy was a soldier for 15 years before he studied physics for 3 years and discovered SU(3) and quarks before Gell-Mann did.

So switching fields opens your mind for new discoveries. If you stay very long in the same field you're mind gets just used to all the unsolved problems.