WELL, AND MY WIFE AND I CAN'T AGREE ON SOMETHING AND YOU WILL BE THE
REFEREE!  WE'RE HAVING A BIG 50TH BIRTHDAY PARTY (~125 PERSONS) AND MY
IDEA WAS TO DISCREETLY PUT "NO GIFTS PLEASE (MAKES HIM FEEL OLD!)" AT
THE BOTTOM OF THE INVITATION, WHICH PHRASE I THINK IS PERFECTLY
ACCEPTABLE AND ELEGANT AND APPROPRIATE, AND WHICH SHE THINKS IS A
TACKY FAUX PAS.  WHAT'S THE VERDICT????

I don't see anything wrong with placing a statement such as "No gifts,
please" on the invitation, but I find the "makes him feel old" part to
be a bit discomfiting.

I think it should be "No Gifts Please , your Presence is all we need!"

Your wife is right.

Wives are ALWAYS right.

Or, at least, they like to think they are.

Just go along with it, man ... Life is a whole lot easier thataway.

I would say it depends on the formality of the celebration.  A black
tie dinner, that would be tacky.  A backyard barbeque, a bit of humor
would be cute.

For my Grandmother's 100th birthday we put something like "No gifts
please, your presence is your gift."

Well another idea would be to have attendes donate whatever money you
want to your favorite charity. But yes if you do not want any gifts it
is tactfull to put a no gift disclaimer on the invitations.

I think asking for no gifts is appropriate.  For my daughter's
engagement party we ended the invitation with "No gifts please...just
your company."

Happy Birthday!

Ask them all to bring a bottle - solves the gift problem, everyone
gets to share the communal hangover.  Have fun!