30. Going to view a possible purchase from a chap that just wants rid of his 'no longer used/loved/wanted bike' and watching his eyes glaze over as you witter on about retro's for far too long.
Never, ever wax eloquently about retro bikes in that situation. You're just there to pick up a slightly-better-than-BSO bike for your <insert relative's title here>. The one in front of you is acceptable (with a bit of work, of course), and you want it for as cheap as possible.
Spot on. There are no ethics involved looked at gem going dirt cheap to begin with, then knocking down the price even more with the above argument. Then
when the deal is done spout on about how your 14 yr old teenager son likes long top 23" top tubes, can't be doing with riser bars, plastic finishes and will be really chuffed. Then say "isn't vulcanisation great" and pop the question if the seller can throw in the retro puncture repair kit and bike lights too as they won't be needed!
I once had a chap who said "I can't believe your wife would be interested in this old crap, and you haven't even ridden it to try the gears and brakes before you decided to buy it".