I tracked down this note about the aging of Númenóreans from Tolkien’s Unfinished Tales which could work:
"It is further expounded that the increase in the Númenórean span was brought about by assimilation of their mode of life to that of the Eldar: though they were expressly warned that they had not become Eldar, but remained mortal Men, and had been granted only an extension of the period of their vigour of mind and body.
Thus (as the Eldar) they grew at much the same rate as other Men, but when they had achieved "full-growth" they then aged, or "wore out," very much more slowly. The first approach of "world-weariness" was indeed for them a sign that their period of vigour was nearing its end. When it came to an end, if they persisted in living, then decay would proceed, as growth had done, no more slowly than among other Men. Thus a Númenórean would pass quickly, in ten years maybe, from health and vigour of mind to decrepitude and senility."
(UT, 'The Line of Elros: Kings of Númenor', note 1)
An adult aasimar wouldn’t change much as they aged, except perhaps they’d have the look in their eyes of having experienced more. That is, until that life caught up to where their mortal “world-weariness” outweighed the celestial blood in them and their body began to quickly catch up with itself. For most, it would be that extended lifespan given in the books. For some, perhaps, living in trying times and confronted with great evil, they might “age” earlier than their peers (as if they’d rolled lower on the maximum age roll).
