There is no specific formula for birthday party invitations past the major milestones of 13, 18 and 21. A birthday card should reflect the birthday person's interests, where she is in life and how her year has been, so individual initiative is needed in writing one.
Rhyming contrivances are popular in birthday invitations. They are not scaled to certain ages or age groups and can be applied appropriately to anyone, making them equally suited to 18- and 90-year-olds.
An alternate strategy is to list the birthday person's name and upcoming age and then provide a brief, enthusiastic description of the party. This gives people an idea of what it will entail and how to dress.