I am over forty and am a management employee for an "at will
employment" company.  I worked for them for 14 years previously and
quit.  I was re-employed 4 1/2 rears ago.  In 6 months my time will be
"bridged" and I will be eligble for a better retirement package.  My
immediate supervisor asked me via email, today, if I had over 15 years
with the company.  Can I be legally terminated by the company to save
themselves that extra future cost?

I don't claim any special expertise in this area, but my general
understanding has been that the answer is probably Yes -- they can
fire you at any time, pretty much for whatever reason.

However, if the company consistently fired older employees (since
older workers tend to have more seniority) then they could be leaving
themselves open to charges of age discrimination, and consequent
lawsuits.

Perhaps this supposition of mine can be confirmed by a researcher
here.  In the mean time, let me wish you luck.

pafalafa-ga