Why do LEGO sets have a minium age rating?
With children 3 and under, small parts may be a choking hazard. The sets that Lego markets to the parents of small children contain larger pieces that are easier to handle and harder to swallow (i.e. Duplo).

When a Lego set recommends "ages 10 and up", it refers more to building ability and patience than safety. These sets are generally more complex and larger than, say, sets an average 8-year-old would be comfortable (or capable) of building. These are only recommendations of course, not rules!