Cathy.. cruise rates can be advertised in a different ways...
'the starting at $699" will always mean that will be the rate for the lowest category on the ship. However, it's IMPORTANT to know if that includes port charges and taxes, or if they are added on.
As for Senior's rates.... the cruise lines don't have any set senior's discount. Occassionally they will select certain sailings of certain ships and offer special seniors discounts, or regional discounts for those living in certain areas.
It would be best to do some research and find a good travel agent who can assist you.