We had a former MH with tires about 6 yrs old. Showed no signs of cracking - until we had it inspected to sell. Turns out there were major cracks between the treads that we couldn't see. We replaced them. Better safe than sorry if you ask me. A MH blowout can be catastrophic not only to the body and components of the MH but to people.
As far as PSI rating, without loading the MH for travel and weighing then comparing to the tire manufacture's chart for proper inflation, the stickers in the MH are only a guide.