Although, I have marked my response as 10k-30k KM age, as the best time to enjoy a car, I feel this is not an honest reply. I have enjoyed each and every stage of my previous cars:
Running In - You enjoy the smell of new plastics. Joy of owning a new car. Congrats, your fling has just starting talking to you.
1-10k: You enjoy the new car feel, get accustomed to the drive. Enjoy the first long trip on your new car. Your fling is now your good friend.
10k-30k: Time for some mods that won't void the warranty. Go fanatic over your car, clean it daily just to attain the showroom look every day. This is the time when I go mad over T-BHP, mad lurking over every possible DIY, Car maintenance and modification threads. Your good friend is now your girlfriend. Love her or let her go !
30k-50k: Fresh out of the 30k service you realize a new level of smoothness and mileage. This is the time when you enjoy tripping the most. This is because you have the reliability of a new engine and performance of a perfectly run-in car. (If you had done it properly till 5k). A worthy life partner in making. A perfect alliance already crafted.
50k-90k : New shoes, new clothes, time for an audio upgrade. Cheers !!! Most probably by this time you would have done away with your EMI's and completed the warranty period. You are free !! You are fearless !! Just take the road, anytime and every time. Get performance mods , get that high end Audio install you were always eyeing at. This is the time when your girlfriend becomes your newly wedded wife. Live your dreams with her.
90k + : This entirely depends on your priorities and reliability of your machine. For me, my Hyundai Santro (1998) was fun to drive even till 2.5 lakh KM (out of the total 2.78 Lakh KM I Drove), While my Maruti Suzuki Swift rattled to death at 95k. Beyond the 90k figures, your life with your car depends on many factors. It cannot be generalized. Perhaps, by this time, your car has either matured into a bickering wife or a lady of wisdom and acquired taste. This transformation is entirely dependent upon the relationship you maintained.Irrespective of what happens, you will never forget her voice in your life, even after you decide to break the alliance. Your car has your distinct smell attached to it. More than the enjoyment its the love that will keep you together after this point.
forgive me if you find the analogy a bit outrageous.