I had the privilege of tasting "The Glenlivet" scotch line. Being a total novice, like most of the people there, I thought the aging was similar to bourbons. It isn't. A 15 year old is not a 12 year old plus 3 years. Each one is aged in different wood barrels which gives each a unique character. I actually thought the 12 year old was "flat", the 18 year old was nice but, the 15 year old was my favorite. The unfiltered "Nadura" was even better, so good in fact I shelled out $48 for a bottle which turns out to be a really good price for it.
Next time you see your father, be sure to look at the bottle and note what wood was used in the barrels. That, in addition the age, will help you find his favorite bottle next time.