It simply sounds like personal stressors. I am the same way. I am in a new job....also in a probationary period and I have made mistakes and am still learning, but I stress about my own work performance more than my coworkers do or my supervisor does. Even when things have gone perfect and we have done very well at a job for years, we can suddenly feel uneasy with different situations coming up.

When I was yougner, I felt c.ockier and more confidence because I was just starting out. Now, at 47, I fear competition with younger employees, physical health, memory loss due to age, etc.

It could also be the opposite. A younger person might feel mroe intimidated, being new to an environment, and in later life, feel more confidence in experience collected.
The 90 day period does not sound any different than other workplaces.