I was given a Proflex 855 by my father,which had perfect looking tyres on it which had been sitting in his hallway bathed in UV light for a decade.When I tried to take them off to fit new inner tubes,the tyre lever punched a hole through the rotten carcase.
I always replace tyres irrespective of wear when the surface shows cracking,ever since the carcase of the front tyre on my 2004 Enduro bulged during a steep descent,the tyres had been on it when I bought it second hand and because they had tread I carried on using them.
I used to caravan and the advice there is to replace tyres every 4-5 years,irrespective of wear,due to general degradation over time
In short I would not use old tyres.