Mamoru's parents didn't actually die in a car crash you see. When he was 12 years old they were all abducted by aliens from planet Zorgelplighentingboo (that's pronounced 'your-gull-dee'. I know it looks a little different than it sounds but remember these ARE aliens here..) They were frozen in an anti-age chamber for 4 years until they were unfrozen and made sure that they got in a car crash on their way home for the first time. Unfortunately for them, Mamoru was asleep and had a pillow along so when they went off the cliff, he had his head on it and was consequently saved. Contrary to popular belief, he had amnesia before the crash because of all the testing that the aliens had done. So he went to the hospital and the aliens made him look like he was 6 years old and that's where he met another 6 year old, Usagi. Then the aliens realized that he was smarter than any other 6 year old so they had to make him 8 then turned the girl 4 because they didn't like her very much. So no one remembered what was going on and they lived like that until Mamoru was 12 again when they decided to abduct him again. So this time they didn't freeze him but taught him all the things about being a neuro-surgeon which is why he's so smart. When he turned 18 they brought him back to earth and gave him some sunglasses because it would take a few months until his eyes got used to the sunlight again and an ugly green jacket to remind him of them. That was the day Usagi threw the paper on him which explains why he was such a freaking loser.

Remember, this is only a theory.