That does seem to be the norm; I've rarely seen Robespierrre portrayed as being the right age; if he's not 75, he's at least 45 instead of 35. I think Couthon may get off rather better here than he does in, say, Gance's Napoléon--at least here he doesn't have the appearance of being completely off his rocker. Which is not to say that I don't object to their making him old (and apparently fat too, which is a worse sin these days, it seems)...