I won't speak to the life expectancy but I image you're fine with a 15 year old system. You will want to have a very detailed inspection performed though to check out the following:
1. Capacity (how many hours/days of capacity do you have if it becomes inoperable
2. Alarm light/buzzer working?
3. Condition of the pumps
4. Condition of the tank
This inspection can run $1,000 or more depending on what you have and you should also request the inspector put together a recommended preventative maintenance schedule.
You should also plan to install a generator ($10,000-$15,000 parts+labor) and an autodialer.