It is all about personal tastes. I know a lot of folks who prefer their VP at 3-10 years of age. They get a real kick (in the teeth) from the abundant vibrant upfront fruit and incredible structural components. The WOW factor can be enjoyed by folks who say, drink their 2002 Merus, Sloan or Insignias this year.
Then there are the Port lovers who like some age on their Vintage Ports and between 10 and 20 years old find the purity of the still ruby based fruit with softer tannins, more interesting.
The majority of Vintage Port lovers like how the mature Vintage Port at 20 to 40 years old drinks. It is here that the secondary characteristics normally show themselves as well as the harmony between fruit and tannins. With the best examples like many of the 1966 and 1970 VPs today, they are still drinking fabulously and some are still improving ... 1966 Dow and 1970 Fonseca are just two great examples.
Some of us (me included) love the 40-60 year old wines that are fully mature in most cases. Almost all 1963, 1955, 1948 and 1945 fit into this category. The best wines from these vintage are normally drinking near their peak of performance and although great, are no longer getting any better.
You've already heard about the next phase in my response above. The one where the tertiary flavors kick in.
After that you have wines that may live on a long plateau and a few great wines like Vargellas, Niepoort and Burmester from 1912 fit into this mold, as do VPs like Cockburn, Dow and Taylor 1927 for example.