st: Creating a mean of x entries in variable1 if there are at least x entries in variable1 by person id

[Nicholas Winters <nicholas.winters@mail.mcgill.ca>]

Sorry for the complicated subject, this is what my data looks like:

person id	exposure	year
1	         3	           1
1	         3	           2
1	         4	           3 
1	         3	           4
1	         3	           5
2	         4	           1
2	         4	           2
2	         3	           3
2	         4	           4
3	         1	           1
3	         2	           2
3	         3	           3
3	         2	           4

I want to take an average of 'exposure' if there are at least, say, 3 entries in 'year' per person id. So I want to end up with a new variable, mean_exposure, that looks like this:
person id	mean_exposure
1	          3
2	          4
3	          3
4	          2

I've tried by person id: egen mean_exposure=mean(exposure) if _____________ 
but I want the mean of ALL values in exposure only if 'year' has at least x number of observations 
please help, I'm stumped...?
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