A 90th birthday party invitation should include the name of the guest of honor, the date and time of the party, the location, and contact information, such as a phone number or email. This gives all of the guests who are invited the information they need to attend the party, as well as a way to contact you should they have any questions they need cleared up.
Depending on the setting, you may also want to consider implementing a dress code, such as "casual" or "black tie optional." This clears up any confusion people may have about the tone you want for the party. If you need to know who is coming, include an RSVP deadline.
If other people put significant effort into your party, you may also want to list them as hosts or hostesses, as a way of showing your appreciation.
Timing is just as important as what goes into your invitation. For best turnout, you want to send the invitations out about a month before the date of the party. The more formal the event, the earlier you should send the invitations. This gives guests all the time they may need for preparation or the rearranging of their schedules.