Near the beach 10 years is pushing it for the outside unit.
Beachfront, or very close to the beach, 5 years is the rule of thumb for an outdoor unit. Ten if you're a few hundred yards back, the unit is sheltered (i.e. on the opposite side of the house from the beach) and you periodically hose off the fins of the coil to remove salt buildup (hose gently...do not use high-pressure or you might remove the fins themselves if they are old enough).
Air handlers (the indoor unit) are less of a problem since they're inside, but if you are replacing a unit that is 5 or 10 years old, then you will likely have to replace the air handler as well. The industry was forced to switch from R-22 type freon to R-410 about a year or two ago, and an R-22 air handler cannot be attached to an R-410 outdoor unit, so both pieces typically have to be replaced.
There are a few dealers who have a handful of R-22 outdoor units left, but they're going fast.