Please have a look on the following sentences:
- As I get older I get more optimistic.
- While I get older I get more optimistic.
In the example above, using 'as' or 'while' does not make any difference in their meanings at all, but in the following sentences:
- While I am getting older I am getting more optimistic.
- While I was getting older I was getting more optimistic.
I think whereas we have used progressive tenses in both above sentences, we cannot use 'as', because I think 'as' can be used only in simple tenses and 'while' can be used for both simple and progressive tenses simultaneously. Am I right?