What are the differences between these phrases and how to use them properly? Thanks
As I get old
As I get older
As I am getting old
As I am getting older
While TRomano's answer here is quite correct in a general sense, in idiomatic usage the meaning of these two phrases is distinctly different. The difference comes from using as with different meanings in each case. (Among several other meanings, as can mean either while or because.) Have a look at these:
As I get older, I find that I care less and less about little things.
This means that part of the process of aging is that little things lose their importance to me. As in this case has the meaning of while.
As I'm getting older, I think it's time that I quit mountain climbing.
This means that I'm thinking about quitting mountain climbing because I'm too old to do it. As in this case has the meaning of because.
The nuanced difference between the simple present and the continuous is one of emphasis; the continuous conveys the sense of action-in-progress. The difference is much diluted when the verbs themselves relate to the passage of time.
So, "as I'm getting older" would convey the idea that the speaker is aware of the fact that he has been aging and is continuing to age; whereas "as I get older" would convey the idea that the speaker is aware of the fact that he has aged and that he will continue to age.
Not much difference.
On the other hand, if the verb is not directly related to time, the difference can be much more marked:
My dog gnaws on marrow bones with much enjoyment. [it happens whenever he has a bone.]
My dog is gnawing on marrow bones with much enjoyment. [it is happening now.]