My group is about to come across a haunted sunken keep with an adversarial Ghost mini-boss. Being mostly composed of Humans (because who doesn't want a free feat?), I expect them to be particularly vulnerable to Horrifying Visage's aging effect.
The problem is, based on this question and my own reading of the PHB, it would seem to be that age is simply fluff. You can roleplay being any age you want and not have consequences mechanically.
The tone of the campaign is serious and leans toward deadly (though no one has died. Yet), and considering that you can only be prematurely aged by a Ghost by failing a save by 5 or more (a total roll of 8) the effect of aging should be a nuisance, at least.
I understand that 5th edition has done away with giving bonuses and penalties for old age as previous editions used to, trusting the GM to keep the tone of the game intact. For example, it would be up to the GM whether he would allow a Venerable Fighter to still function 100% normally, if that's the kind of game you're playing.
However, it doesn't work for me. The venerable fighter has had years to perfect his swing in order to avoid that ache in his elbow, let alone avoid throwing his back. A magically aged character (or any aged character, if you're running a realist game) who has prematurely aged as a result of effects such as Horrifying Visage has not had the years he needs to get used to the effects of aging and neither has he grown wiser or more intelligent.
How can I translate premature aging into a mechanical effect?
I am open to house-rules, but please back it up with experience of use in-game. Wild speculation and spit-balling are not good answers.
Best Answer
Ah yes, the effects of age, I could tell you a lot about that. But I’ll try to keep focused on in-game effects.
“I’ll regret this later”
Penalties to abilities that simply make the character weaker (like those found in older versions) are frustrating for a player, and they are bland, and don't really evoke the feel of old age. These rules simply never provided any fun, or much depth. It’s no wonder they didn't come forward to 5e.
Effects that limit the endurance a character has, that come into effect after some initial exertion, evoke the effects of age more poignantly, while letting the character still “relive past glories” in brief but glorious combat.
So very tired
The rules for exhaustion in the DMG can be utilized to simulate the fatigue of advanced age, and they come pre-play-balanced for you. A venerable character (depending on age, at the discretion of the DM) may gain a level of exhaustion from a single combat, and require a short or long rest to recover from it.
I just can’t seem to rest like I used to
Speaking of rest, an older character needs more. Simplest thing would be to double the amount of time needed to attain the benefits of the rest, but it doesn't add much color to the game, if the players are simply obliged to say “OK, then we rest for 16 hours.”
I find it’s better to “nerf” the effects of the rest, granting back fewer hit dice, hit points - or even fewer spell slots, if mental ability has been affected. That allows the oldster(s) to “try to keep up” while providing a nagging reminder they are really too old for this sort of thing.
(If you ever played 4E, this was like a non-heroic NPC taking a rest. Unlike the PC’s, the NPC would not wake up in the morning fully healed of all wounds.)
Another nice surprise
I would also suggest that these or any effects of old age should come as a surprise (especially for the prematurely aged). They always do.
My (game) experience with old age effects
I’ve used these rules for adjunct NPCs - I haven’t prematurely aged a PC. The players thought they were fair and interesting rules. It accentuated the power of the (young) PC’s, without making the NPC useless. The oldster slept/rested while the characters did ancillary stuff. It might be different for a PC. I suspect a player won't be overjoyed with having to “take it easy” but it's similar to other “curses.”
Since initially answering this, I spoke with one of my players about the age rules we used. She commented the rules made her feel protective of the NPC.