Most of the 7.5 R's I see have factory fitted Continental Contact Sport's in 235/35/19, so id say your rears are the original tyres (going off the tyres manufacturing date vs your cars manufacturing date).
If the car has done mostly highway driving then yes the tyres could last 50,000km or possibly even longer.
For context, I bought a 2015 Golf GTI Performance about 18 months ago. It had 47,000km on it when I got it. The tyres were the original 225/35/19 Dunlop SP Sport Maxx GT (manufactured 2015) but were absolutely shagged and due for replacement which I did within about a week of buying the car. They were almost completely bald, rock hard, had nicks and cuts in them from hitting gutters and had no grip in either the wet or dry. This car I bought had spent its first few years in Sydney, and then a few years in Newcastle - so mostly city driving.
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