Honestly, the best tip is observation. One of my 2 main characters from my series:
(sorry about that plug, but I wanted to support what I'm saying)
Moe is a senior citizen himself ( at least with the beginning, which is near the end, technically of this new season), and because of this, many supporting characters are as well, so I've become obsessed with designing him and others over and over to get the desired look so I can give you some of what I learned.
-Don't be afraid to literally look up exact age and race references on Google, I do it all the time to be accurate and knowledgeable since different races age different due to melanin and whatnot.
IN LIFE:
-Depending on what age you're going for, around 50+ especially late 50s and up, the face begins to fill out,(sagging cheeks, turkey neck, no matter your weight.
- Upper eyelids begin to sort of droop (not in the same way all the time)
-Hair is almost always completely white at around 70+ (depends on race though)
-Beard hair begins graying pretty early depending on weight, health, and other things, so small strands of white hairs (if wanted can appear as early as mid 30s usually)
DESIGNING
-For designing ELDERLY people as an artist, posture is a much needed detail that needs attention.
-At about 40, you typically see artists add small bags under the eyes and crows feet when smiling (See Shanks from One Piece, very good 40 year old design)
-At about 50, there are more and up to mid 50s, you still don't have exaggerated posture, but more bags, parentheses around the mouth and maybe receding hairlines.
-Pay attention to hands, arms, neck having creases and folds.
-Nose and ears keep growing into old age, so play around with sizes. Play around with lips too.
-Speaking of, lower lips on the face make a character look much older, along with parentheses contouring the shape. On a skinny head, more tightened lips, a longer head is common for up tight or frail old people and bulldog like cheeks with a more extended face is good ( I use this for Moe) for people with more weight., which also works for uptight or sad sack old people (again, I use this)
This might be a bit much, but I've been designing old character a lot lately. Hope any of this helps.