Battery life isn't measured in years, it's determined by discharge/charge cycles at a given Depth of Discharge (DOD). The higher the DOD, the fewer cycles the battery will provide before failure.
A battery routinely discharged to 60% or more will probably only provide half the cycles as a battery that regularly sees 40% DOD, with the caveat that the charging cycle is completed to a full charge. There are so many variables that affect battery life that without extensive testing, it's almost impossible to determine battery condition at any given time. The age of the battery is essentially meaningless. That explains the wild variation in battery life as reported dockside measured in years.
If you replace a single battery in a bank, the replacement battery will surely have a lower internal resistance than the existing bank, so when it's put into the circuit, it charges at a higher rate than the remaining batteries. It overcharges. The remaining batteries undercharge. The new battery will heat because it's absorbing more amperage than the rest, and it will gas off more than the rest. It creates a very unpredictable set of circumstances that rarely work out as planned (to incur the cost of only one battery vs. a whole bank).
If budgetary concerns dictate that you replace only a failed battery in a bank, IF you can find a used battery that approximates the condition of the existing bank it's replacing a member of, you may be able to see the bank through the remainder of its anticipated life expectancy, but it's a crap shoot at best. But you'd be far better to replace a single battery with a used one, NOT new! So you're further ahead with the budget as well as your chances of bank survival with a used battery!
So, all that being said, keep in mind that a single battery failure in a bank is not typically a rogue, it's usually a sage, as it's telling you there's something wrong in the bank, and if one fails, chances are another one will be following soon, so think a bit before replacing that single battery! Given that you found a bad connection, you may be able to correct that and the affected battery may be salvageable.
To buy some time, you might be able to take 2 batteries out of the parallel and run on 2/3 of the bank if you can tolerate the reduced capacity. Either way, check your series/parallel wiring in the bank to insure that all the jumpers are precisely the same length, same gauge, scrupulously clean terminals, and that the main pos & neg connections come off opposite ends of the bank with respect to the length of the circuit path.