Personally I would not exceed 7 yrs on a MH tire based on the manufacturer's date.
How old are your tires actually? If you haven't checked, look for the manufacturer date on each tire. It is on the sidewall of each tire, a 4 digit number following DOT, like 2313. In this case, "2313" means the tire was made on the 23rd week of 2013.
Also, how did you determine that the recommended tire pressure is 90 lbs? If you look on the wall below your drivers side window, the manufacturer's recommended tire inflation for front and rear tires will be listed on the manufacturers sticker. Often is different for front and rear tires.