If it is at a place where the menu is online direct people there.
Another thing to check is will they let people pay separately. Went somewhere recently and they wouldn't allow this.
I cannot think of the wording but one invite said no presents as guests would be paying for their meal, drinks were able to be purchased at the bar. Something like that.
I think as it's older people coming I would mention that they are paying for the meal as I know it's something they are not used to and will save them being embarrassed at the end of the night.