To the extent that funds are conveniently available to the city, a qualified senior citizen may choose to have deferred the special assessment for street, curb and gutter and related improvements that is to be levied against his or her residence. This assessment shall bear interest at the rate of 7% per year from the date that it is levied against the residence and until paid, and shall become due when the property ceases to be the residence of the senior citizen or his or her spouse. This may be by reason of the sale or rental of the property or the death of both the senior citizen and his or her spouse or for any other reason whatsoever.