Long term aging of mead leads to a very mellow mead. It can really bring out the honey's profile. However we must remember that Oxygen is not our friend when making mead. Oxidation can literally ruin the flavor profile of mead. With that we must further understand that cork is not an oxygen barrier. so when long term aging of mead (anything over one year. we must consider a stopper that provides protection from oxidation. Always use synthetic corks. and add Potassium metabisulphite at a rate if 1/4 tsp per up to 6 gallons of mead just before bottling. I have meads that are in excess of 25 years old. We refer to them as Special occasion meads.
I am very late to this party, but I brew meads (& wines & beers) intended for long-term ageing and can maybe add some advice. The rules for all of them are similar, although mead can offer some additional difficulties - largely due to a relative absence of natural anti-oxidant capacity.
As with any mead you want to start with a quality honey, and this is even more so with longer aged meads as meads made with lower grade or adulterated honeys tend to loose flavour more quickly. These meads are also a great opportunity to use strongly-flavoured honeys that don't normally make nice meads (e.g. buckwheat), as the flavours will mellow into a nice beverage over time. Spices and fruits can be added as you see fit, but keep in mind that delicate flavours are unlikely to survive long ageing - so a bold fruit like raspberries or cherries is a good choice, as would a bold spice like cloves, but gentler flavours (e.g. strawberries, saffron) are unlikely to hold up to years of ageing. Acid blend can be use to balance acidity; I've heard (but never experienced) that wine tannin will settle out over several yeast, so adding it for body may not yield any benefit.
Gravity-wise you want to aim for the high end; 12% ABV minimum for stuff you want to age more than 5 years. Typical ranges are 4-6lbs honey per gallon; S.G. of 1.120 to 1.140. Some go even higher. These high gravitates will often not ferment to completion, but that is OK as the residual sugar will help in the ageing process. My personal preference is to not pasture the honey or boil/heat the must, as this can drive off a lot of the honey's character. Whole fruits I will wash in the sink with a cup of peroxide, and then quarter and freeze before adding to the must. Spices I just toss in. Add lots of yeast nutrient (there are good multi-step nutrient addition schedules available on the 'net - I recommend these fully), oxygenate well, and pitch a lot of healthy yeast (2-3 packs of properly rehydrated wine or mead yeast, per 20L/5 gallons). Ferment to completion (3-4 weeks) and then transfer to a carboy with airlock for secondary ageing. Note that after the first day or two you want to avoid introducing additional oxygen into the must, so avoid stirring or splashing the fermenting mead, and perform all transfers with a siphon or spigot/hose.
How long you age in secondary is upto you, although a few months is the minimum to allow for clarification. When ageing be certain to minimise headspace - if there is more than a few inches of headspace top-off the carboy with boiled/cooled water, or with a complementary wine/mead/cider/etc. Be sure to keep the airlock filled - oxygen at this point is your enemy. I typically age for ~6 months, and then transfer the cleared mead to another carboy for an additional 1-2 months; this second step is not necessary, but does help with clarity. You can stabilise the mead as you would for a wine (metabisulfate + sorbate), but I don't usually bother with this myself. Do not use one of those cheap pad-based wine filters as they are notorious for introducing oxygen into your mead (or wine), and can scrub out some flavour; simply let time and gravity clear the mead for you.
Once you're happy with the mead its time to package. I would suggest packaging these still, as carbonation often doesn't last through years of ageing. Transfer the mead to a clean bottling bucket or carboy, being careful not to transfer any yeast/trub. Clean and sanitise your bottles; if you have a CO2 system you can pre-purge your bottles (in thoery this should further reduce oxidation, but none of the meads I've packaged this way are old enough to see if it helped). Fill each bottle, minimising splashing during the fill, and leaving only enough empty space for the cork + 1-2" of headspace (if using crown-capped bottles, fill to leave 1-2" of headspace). Cork the bottles and leave them upright for a day or two to let the pressurised air in the bottle leak out. Then wax the corks (helps limit oxidation) and store on their side (to keep the cork moist and well sealed) until you're ready to consume. Crown-capped bottles should be stored upright to limit corrosion of the cap. You can add between 1/8th and 1/4 of a teaspoon of potassium metabisulfate at bottling time; this will add as an antioxident and aid in long-term ageing. My personal preference is to do this with meads containing fruit or spices, as it seems to help preserve those flavours; for straight honey-meads I don't bother - many of the oxidation characters that appear work well with a straight mead, leading to a mead with sherry-like characteristics.
Tenbears has meads with a few years more than mine - my oldest that I still have on hand was brewed for Y2K celebrations, meaning its now ~20 years old. Its a straight mead and has only gotten better with age. I have a few melomels (fruit meads) nearing 15 years old, but they are definitely past their prime. I don't brew many metheglins (spiced meads), and I've found most are limited to a few years - but I also tend to favour lower gravities and more subtle flavours with those styles. But with a bit of care, and a high starting gravity, its not too hard to make a mead that'll easily last a decade or more.
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