Guidelines will be published by Thame Youth Memorial after a mother spent more than a year thinking her son would be remembered there, only to discover he will not.
Diane Thompson, of Chinnor, believed her son Robert, who died aged 29 in 2010, would get a plaque at the Upper High Street site.
The memorial group denies any promise was made, saying it has always operated around the principle that youths aged five to 25 would be remembered there.
Mrs Thompson was under the impression her son would be included after a conversation with treasurer Mike Dyer.
Mr Dyer said: “I made encouraging noises that an application would be considered, but I didn’t say it would be accepted. I would have been in no position to do so.”
Mr Thompson was fatally injured in a crash in Bicester. Mrs Thompson said she wanted him to be included in the memorial so his four-year-old daughter Kyra would have somewhere to go and remember him.
Mrs Thompson: “I wanted her to be able to concentrate on the memorial rather than the roundabout where he died. I feel like I have let him down now. I just don’t want the same thing to happen to anyone else.”
When Mrs Thompson and Mr Dyer first spoke about the memorial there were no guidelines available publicly about age limits, but the group says it will now publish something on its website.
Peter Butt, the memorial’s chairman, said: “There has not been a change in the criteria but it has perhaps not been clear enough. We will publish it all on our website so this can never happen again.”
Mrs Thompson has been told she can have either a bench or tree in her son’s memory, but will have to pay for this herself.