this does not make much sense "your physical age is reduced by 1d6 + 6 years, to a minimum of 13 years."

It means that the potion can't make you younger than 13 years.

For example, if you are say, 18 when you drink it, even just the assured +6 years reduced would make you 12 years old physically, but the hard cap is that no, you will only be de-aged the 5 years until you are 13 years old. Hence there really isn't a point to drink it when you're still that young, as is sensible.

More obviously, it's evident that this restriction was put in place because, surprise surprise, neither the people who created this potion, nor WoTC themselves wanted to promote or deal with the literal possibility of you turning yourself into an actual child, and then continuing to adventure as such, because Lord knows the messy controversies that would arise from that...

Hope this helped!

That was great thank you, that made me laugh.

Happy to be of service!~^^o7

So you roll a d6 and add 6 and take that away from your current age i.e. rolled a 5 so you subtract 11 years from your current age. But you cannot become younger than 13 years old

Quote from BrothersintheCreed >>

So you roll a d6 and add 6 and take that away from your current age i.e. rolled a 5 so you subtract 11 years from your current age. But you cannot become younger than 13 years old

 Pretty much. Most the time the party is going to be in their mid 20's early 30's more than likely. With the exception of Long lived races. It's a to give the Older PC's some more years.

Does this have any use to player characters other than being a potential plot hook?

Note that it doesn't specify a WILLING creature. Do with that as you will. 

If one got aged up/started out old ie wise old wizard then they could get some more time + short-lived races get some more time

10% cumulative chance means that from dose 11 on, the potion will always age you older. Interesting.

/deleted/

Quote from trapbuilder2 >>

Does this have any use to player characters other than being a potential plot hook?

 Counteracting ghost aging. I just used it on my fighter who had aged 20 years from a ghost encounter at early levels before they could get it restored fast enough.

Could make for a great story. Gather the ingredients; for a king, wizard, loved one...is this taboo? Normal in your setting?

We have an elf in our party that got lost in the Astral Plane as part of their backstory and is 700 years old.  I could see him consuming lots of these regardless of the 10% risk

This is perhaps the most useless magic item in the game. Assuming you have a wizard in your party, (or anyone who can use Wish to cast a spell from another class list) you've already got access to the Clone spell which basically just gives you immortality and the ability to be a younger age anyway. This potion can give you AT MOST 12 more years of youth. Clone can just make you a young adult again when you die and protect against death in general. Very rare items tend to cost in the range of tens of thousands of gold, even consumable ones; and by the time the party can freely spend that much gold on a gimmick like this they're already in the power range where better options are available. It's made even more useless by the fact that every time you drink one, the next one makes it increasingly more likely to have the opposite effect. Flavor-wise it's cool, but practically who would ever use this?

how did you manage to get your hands on a very rare magic item at early level?

Quote from DoyleClan >>

This is perhaps the most useless magic item in the game. Assuming you have a wizard in your party, (or anyone who can use Wish to cast a spell from another class list) you've already got access to the Clone spell which basically just gives you immortality and the ability to be a younger age anyway. This potion can give you AT MOST 12 more years of youth. Clone can just make you a young adult again when you die and protect against death in general. Very rare items tend to cost in the range of tens of thousands of gold, even consumable ones; and by the time the party can freely spend that much gold on a gimmick like this they're already in the power range where better options are available. It's made even more useless by the fact that every time you drink one, the next one makes it increasingly more likely to have the opposite effect. Flavor-wise it's cool, but practically who would ever use this?

Sure, mechanically it does basically nothing. It will, however, be valuable and sought after in almost every setting-think about how much people would pay to be younger. Based on this, I think the primary use of this is as something a character could sell for a lot of money.

Quote from GandyTook >>

We have an elf in our party that got lost in the Astral Plane as part of their backstory and is 700 years old.  I could see him consuming lots of these regardless of the 10% risk

 The thing is, it's not a 10% chance each time, it's a 10% cumulative chance. This means the 3rd time it's 20%, 4th time it's 30% ETC

Gotta say, for the short lived races (especially those aged by magic) this is an invaluable item. Definitely want it on my Thri-kreen after he got aged by a ghost to 54yrs old!