According to Seeker :
What's the common wisdom on this? Reshingle or tear up the old
shingles and do a new roof?

On a house 21 years old, with the existing asphalt shingles in reasonably
good physical shape and no existing damage, there's no particular reason to
_not_ simply lay on another layer of shingles.

You start thinking differently if:

1) if it'd be a third layer (many building codes prohibit three).

2) you have roof leaks, and/or you want/need to upgrade/install
new tar paper or waterproof membranes.

3) A lot of shingles are broken.

4) You have problems with the roof sheathing (ie: rot).

It's a compromise. A tear-off is _clearly_ better - but usually a
lot more costly. For a roof otherwise in just fine condition, a tearoff
might only gain you a year before the next reroof.

If the cost of tear-off is minimal, go for it.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.