Wine increases its strength and flavor, and its mildness and mellowness, by age, and the old is therefore preferable. They who had tasted such mild and mellow wine would not readily drink the comparatively sour and astringent juice of the grape as it came from the press.
The meaning of this proverb in this place seems to be this: You Pharisees wish to draw my disciples to the "austere" and "rigid" duties of the ceremonial law - to fasting and painful rites; but they have come under a milder system.
They have tasted the gentle and tender blessings of the gospel; they have no "relish" for your stern and harsh requirements. To insist now on their observing them would be like telling a man who had tasted of good, ripe, and mild wine to partake of that which is sour and unpalatable.
At the proper time all the sterner duties of religion will be properly regarded; but "at present," to teach them to fast when they see "no occasion" for it - when they are full of joy at the presence of their Master - would be like putting a piece of new cloth on an old garment, or new wine into old bottles, or drinking unpleasant wine after one had tasted that which was more pleasant. It would be ill-timed, inappropriate, and incongruous.
[Excerpt from Barnes' Commentary]