If someone’s 18-year-old car is in mint condition, it has been very well taken care of and is still almost as good as new. Anyone looking at it would think it was new (except that it's probably out of style).
The term mint condition originated in the field of numismatics — the study and collection of coins. A mint is a place where coins are made for use as currency. A freshly minted coin was described as being "in mint condition" — free of the scratches, dents, or wear that comes from being in circulation. All kinds of other things might now be described as being in mint condition: stamps, bicycles, clocks, books, cars, etc. Ironically, however, it usually refers to things that are used, not brand new, but that have been kept in near-perfect condition.