Bobby and Wendy got out to his car and headed for his condo.
" Thought any more about how old you want to be?"Bobby inquired curiously.
"Well, I could have you just make me 21, but where would the fun be in that. Tell you what, let's just get to your place, and then consider the possibilities together. Plus, I have a little surprise for you when we get there, as my own way of thanking you for being such a good friend all these years."
What happens next?
Bobby and Wendy have wild sex, and then he makes her 21.
Bobby and Wendy have sex at various ages for Wendy, he ages her each time 10 years until she is 65, and then he gets ready and they head to TGI Fridays.
Wendy turns 40 they have sex,an then they check out the book to find out what the ring can do, and make plans accordingly.
Some combination of the above.