I found this when searching, it came up in a mass for a teacher's retirement, but I think this could work for any retirement. It isn't really a prayer, but since it was used inconjunction with a retirement mass, I thought it might be appropriate.
Congratulations to you aunt, I hope she enjoys her retirement very, very much!
This is where your life has arrived,
After all the years of effort and toil;
Look back with graciousness and thanks
on all your great and quiet achievements.
You stand on the shore of new invitation
To open your life to what is left undone;
Let your heart enjoy a different rhythm
When drawn to the wonder of other horizons.
Have the courage for a new approach to time;
Allow it to slow until you find freedom
To draw alongside the mystery you hold
And befriend your own beauty of soul.
Now is the time to enjoy your heart’s desire,
To live the dreams you’ve waited for,
To awaken the depths beyond your work
And enter into your infinite source.