Customer Lifetime Value $($LTV$)$

Assuming that the average customer will use their smart phones until they are $85$ years old, how would you calculate the lifetime value for each of the following customers? Assume that their average monthly smart phone cost will remain the same throughout their lives.

$\backslash$table$[[$Customer$,$Current Age,$\backslash$table$[[$Monthly$],[$Cost$]]],[$Shelly$,25,$$$80$