It's exponential, really...
Let's get one thing straight first. At The Rum Project, Sue Sea and I are in deep, messy love with Ron Matusalem Gran Reserva 15. In fact, we long ago made the 15 our reference rum for the cuban style category. The 15 is truly one of the great rums, and really an amazing value.
But let's get to it...
The Gran Reserva 15 is a solera rum, one of least defined methods of aging, but in general consists of a number of levels, typically three or four levels for rum. The system begins with all the levels being filled with new make. After a period, say 6 months or a year, a certain quantity, say half a barrel is withdrawn from the bottom level, and replaced by rum from the level above it, which is replaced by the next highest level and so forth.
In this way, rum progressively moved down from the top level, level by level to the bottom level - where it is finally removed for bottling. There are two important facts about soleras:
1. A certain amount of rum always remains in the bottom level. If the solera is say 15 years old, then you will find some 15 year old rum in the bottom level, along with much younger rums from the higher levels.
2. The average age of the rum in the bottom barrel is much different. Spreadsheet analysis reveals that solera aging is exponential, and that the average age tends to max out at about 8 years, despite a longer solera. In other words a 15 year solera may average 7.7 years, while the 18 year solera may only add a tenth or two, to say 7.9 years.
Not much, and really not much difference in profile (unless you can taste a teaspoon of older rum in the bottle).
Now here's the big myth. Let's again consider the Gran Reserva 15. Once the solera is established, the next year you actually have some 16 year old rum in the bottom barrel, the next year some 17 year, etc. The solera always gets older. Keep in mind that the Gran Reserva 15 has been around for a some years. The truth is that the solera should really be renamed each year. Gran Reserva 15 - 16 -17 - etc.
But no company really does this. The Reserva is actually getting older each year, but keeps getting sold as a "15". So what has happened? Whether Matusalem added a level, or cycles the some of the bottom barrel to another sytem (like Zacapa), one thing is for sure: the average age is not much different.
At The Rum Project, Sue Sea and I have compared the 15 and 18, and honestly, the difference is not really remarkable. As a result, it's really hard to justify paying more for the misleading "18" or "23", based on fairly trivial differences in average age. In the world of non-solera aging, another 3 or 5 years really means something.
Not in a solera. The Gran Reserva 15 is truly a great rum already. Save the 18 for your ego or a fat wallet.