Interesting ideas on this thread....
It caused me to start googling and I came across this tidbit.

"The number of moles a person has was found to have a correlation with telomere length.[8] The length of telomeres may be of significance in the ageing process. But the relation between telomeres and aging remains uncertain.[9]"

From here, under 'signs and symptoms'
[link to en.m.wikipedia.org]

I'm not really sure how it relates to your original query about birthmarks.
I'm not sure that moles technically qualify as birthmarks.
But I thought it was interesting and so I'm sharing....