Interesting. My mother has a clump of birch trees that is 30 years old. I
remember it being planted when I was pregnant with my first child who just
celebrated her 30th birthday.

I doubt these are exceptions. I remember birch that were that old all over
the place.

What sorts die at 15 years?

Clump Birch die young. They are multiple stemmed grey birch sold as lawn
ornaments. Betula populifolia. The youth and disease comment was pretty
accurate. The comments regarding other birches would need to be directed to
the specific species of birch discussed. All kinds in the US plus many
imported species. Lots of times on this forum missing location of plant and
species or accurate description gives the questioner a wide range answer though
the respondent is trying.