Telling Co-Workers

I was part of a baseball(slow pitch) team last year that was
organized by the company I work for. My PA didn't really effect me
that much(back then) except when I had to run. Anyways I had a
conversation with a buddy from work. I told him I am not sure I can
play beacause of my knees.
His response was "Come on you are the youngest person in the
company ..."
I have told VERY few people at work about my PA. I say that my knees,
and hands are 'BAD', but never saying "I HAVE ARTHRITIS"
I can't stand pity and I can't stand people that say "...you are too
young for that!"
I am battle this disease by myself and I will not let it defeat me.
I would like to play but currently I am between drugs and trying to
get my PA under control.
Has anyone had experience of telling co-workers about PA???
If so how did it go?
Cheers
Shane