Our sun is a G class star - current estimates suggest they make up about 8% of the galactic population. There are other classes such as F and K that might be candidates, but I think 8% of 200 billion is probably enough to work with.
The age of a star is fairly irrelevent in the debate - Instead we need to look for what are known as population I stars. These stars formed with elements heavier than helium, thus allowing planetary formation, and consequently then the basic building blocks for life to accumulate.
Unfortunately I have not found any definitive data on numbers of this population. But in essence anything outside the galactic bulge should qualify.
Either way there are a bucket load of oppotunities for life to get started in just our galaxy - In the whole universe.... the numbers would be mind boggling
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