Check the manufacture date on the tire sidewall. The trailer may be 5yrs old but the tires may be 6-7yrs old if they sat around for a while first. At 5-6yrs I would be looking at replacement.
If you are comfortable with bearings and brakes on a car, should be no issues doing it on a trailer. Youtube a few how to and have at it. This will also give you a chance to check the brake conditions. Don't forget the magnets on the brakes.
You should be checking and touching up the roof seams annually (6 months would be even better).