Well I don't know that it needs to be 20-30 years but if you can get it to last that long, it should taste great.
I have noticed that 3-5 years seems to be the norm though.
Brian have you ever tried the JOA recipe (Joe's Ancient Orange). It is an excellent recipe for a beginner, well anyone really, and takes around 3 mths from pitching, to a very drinkable sweet melomel of around 12-13% depending on you yeast. If you follow the recipe it is almost fool proof and a great way to keep your hands off the other meads that you are wanting to age.
This pic is of my 15ltr demijohn that has an apple, date and honey and treacle cyser that is supposed to be aging. As you can see, at 4mths it is very drinkable, so I am in the process of making another 30lt batch that I hope to age for at least 1 year, and see what the difference is, as the changes in the mead in four months is amazing.