i remove the tube
then inflate it to find the hole
and determine and fix the cause of the flat
then i replace the tube with a known good spare
either new or patched

then patch the punctured tube when i get home and have some time

the only occaisions where i do not patch tubes
is when the puncture is very close to the valve
or there is other damage to the tube besides the puncture
like abraisions from spoke heads when a rim tape has failed

but as long as the tube holds air
then it is good to go
regardless of age