Q:

How can PCs find how to restore Horrifying Visage aging?

The Horrifying Visage attack of a D&D 5e ghost involves a Wisdom saving throw with a haunting failure ramification:

If the save fails by 5 or more, the target also ages 1d4*10 years. 

However

The aging effect can be reversed with a greater restoration spell, but only within 24 hours of it occurring.

As a GM, I want my players to discover this if they don't already know, but not to baldly tell them.
Since the clock is ticking and a long rest/travel time might prevent this from being resolved, how should PCs come by this knowledge to motivate getting it fixed? A skill check of a certain DC? Would particular classes have innate knowledge of this?

A:

An intelligence (religion) or intelligence (arcana) check should help a character know the properties of a ghost's horrifying visage.  How you set the DC is up to you, and is highly dependent upon the campaign setting.  Are ghosts a common thing, where many people might know of the effect that seeing a ghost might have upon the victim?  Or are ghosts extraordinarily rare in your setting, with very little chance of anyone having ever experienced this?  
Perhaps, if no character has the skill proficiency, there is a library nearby where they can learn about ghosts in an ancient dust-covered tome.  If the 24 hour time limit seems too drastic or there is no way for the PCs to otherwise discover the method of reversal in that period, you might consider simply increasing the time period.  If you do this, make sure you remain consistent with that time period or are able to otherwise justify the effect's "permanency" duration in some way.
Heck, maybe the event itself turns into another adventure hook, with the PCs going on a quest to search for a "cure" to reverse the aging.  
I would expect that something as drastic as instantly aging anywhere between 10 and 40 years to be an urgent matter, dependent on the affected character's race and starting age.  An elf might not care at all if he suddenly aged 40 years but a 50 year old human almost certainly would.  
The urgency of determining how to reverse the effect will really be determined by that -- and if the PC doesn't care at all, then it's a fun roleplaying moment with little to no impact on the game, otherwise.  

A:

"You pass an old crone on the road. She steps aside to let you pass, and looks at you. Suddenly she shouts 'You too have seen it! Oh yes! When? Just a few hours ago? You still have time then! Run! Run to the temple, they may help you!' ".
In other words, an unexpected NPC is a solution to all your information dissemination problems.

A:

In addition to making a Skill Check, any character who can actually cast Greater Restoration probably knows what kind of effects it is capable of dealing with. If any of your characters have the required class and level, they probably shouldn't have to make a check for it.
As another alternative to rolling Religion or Arcana, you could also go for a Medicine check, as that's the one usually related directly to curing ailments. You might not know how a Ghost works, but you know how to fix the curses they cause.

