Not everyone knows that since 2006 Pluto has actually been reclassified as a dwarf planet or trans-Neptunian object, reducing the total number of "classical" planets by 1, from 9. This in turn leaves us with only 8 "classical" planets, from largest to smallest: Jupiter, Saturn, Uranus, Neptune, Earth, Venus, Mars, and Mercury.
Not only is Mercury the smallest in size, but is also the closest to our sun.
More Interestings fact about Mercury:
"Mercury's rotation around its axis lasts 59 days, and it takes 88 days to move around its orbit around the Sun. Interestingly, 59 is exactly 2/3 of 88. This is not by chance - it is an effect of the Sun's gravitational field on Mercury. It is a similar phenomenon that the Earth's Moon always has the same "face"; the Moon always turns the same side towards the Earth.
This has a most unusual effect on the length of a Mercurian "day" as measured from noon to noon (note that the word "day" is here used to mean "one daytime period + one nighttime period", corresponding to "24 hours" on Earth). In fact, such a day on Mercury is twice as long as a Mercurian "year" !" Source
A 20 year old born on mercury would actually only be 10 mercurian days old!