Some ADCS scale very well into the late game, particularly ones that incorporate %HP damage in their kits. At the very late stage of the game, it's best to run with Randuin+Frozen Heart to decrease the amount of shots while they are able to throw out. These champions scale very well with life steal as well, so bear in mind that an AP-based tank might be the best choice against late-scalers for sake of additional burst damage--their primary weakness--and Morello's Grevious Wounds effect. If you can't, one of the AD Champs on your team should really be running with Mortal Reminder, or at least
. High armor will only protect you up until the point that the enemy marksman gets a Last Whisper up, and not until you have Randuin+FH and possibly Thornmail up. Having a big HP pool is also an obvious asset.
Tanking an ADC who has left lane is only half the battle--your botlane must also do their part. Late Scalers typically have poor laning phases, so pushing your lane constantly and denying them CS, as well as throwing out strong pokes, will do you a great service come midgame when the enemy ADC can't capitalize on the momentum from their CS to fight. ADCs with strong midgames, like Corki and MF, are very effective at oppressing the marksman in question, as are supports with good pokes and the ability to lock in kills or harass in some other way.
Earlygame oriented ADCs have the potential to get a huge lead on the enemy early on, but if the enemy plays it safe they will outscale you and, at that point, you're screwed.