In my late middle age I've topped 50mph on all my recumbents, AZUB-4 recumbent bike, with front fairing, ICE QNT trike, with the same fairing, Hase Kettwiesel trike, again with the fairing and a naked Catrike Trail, yet another trike. In no case did I get much over 50mph, because, having seen the 50, I touched the brakes. In all cases I was going down a goodly hill. On the AZUB and Kettwiesel I had a tail wind, the other two were on steeper hills.
These days I'm backing off if I top 40mph.
We tend, in these threads to disregard the risks, but 50mph with your bottom 8 inches off the road is seriously quick. Hitting anything is likely to be terminal, even feet first! You need to:-
1. Know the road, in that there are no junctions or drives that something can turn out of having seriously misjudged your speed.
2. Know that the surface is in good condition.
3. Know your bike/trike and how it reacts to braking at that kind of speed.
On a bike and especially an upright, with all that extra drag compared to a recumbent, you have to remember that a speed wobble could be your downfall, literally.
Now nearing 70 I take it a lot easier. Which is my excuse for total lack of speed, anywhere.