If the roof is cedar and properly installed it should last 50 years or more. But those two conditions make a lot of difference. Old cedar roofs were laid on "skip sheathing" meaning a series of horizontal battens nailed to the rafters having centers approximately the same distance as the length of the shingle exposure this was done to save wood of course but it also helped with air circulation and venting. If cedar shingles are nailed to plywood underlayment with roofing felt, they do not last so long.
Also check to make sure there is only one layer of shingles. This is hard to do at the drip edge (closest edge to the ground) because there always is a starter layer. So it looks like 2 or more layers. It's best to check at the roof edge along the gable (pointed end). If you see 4 layers of shingles at any one spot you have two roofs, one laid over the other. This is not necessarily a bad thing. But most building codes do not allow a third layer (too much weight for the roof structure) so if it goes bad you have to tear-off to the decking which adds to the cost.
Cedar roofs are great, they're different than asphalt or fiberglas shingles though. Personally if I had to put a new roof on (which I did myself 4 years ago) I would put architectural shingles on and I'd get the best I could afford (45 year at least).
But if they're not leaking you might have no trouble. How is the attic vented? Gable vents? Small windows at either gable end up high, near the peak. Venting the attic is important because lots of heat builds up in the summer and can cause the cedar to cup and break down.
As for the inspection being only on the inside of the house, I have never heard of that. The mortage company has an interest in the inspection because they want to be certain the collateral value of the property equals or exceeds the amount of the loan, you have an interest because you want to know any hidden problems before you buy. I'd ask whoever does the inspection to check the roof and gutters and venting.
DC
