I assume we're talking about Sims 2 and not Sim 1.
The basic game is rated T because of some more mature elements. Sims are virtual people that can have careers, raise families and realize other lifetime goals. There are 6 life stages with the basic game: baby, toddler, child, teen, adult, elder. Teens and older can make out with each other, which means you see them kiss and run their hands down each other's backs. Teens can only date teens. Adults and older can Woo Hoo, which is another term for sex. You never see them doing this. You'll see covers flying on the bed, arms and legs splashing in the hot tub or the clothes changing booth bouncing around. Sims can get pregnant from Woo Hoo if you click on the Try For Baby option in the interaction menu. This means they'll Woo Hoo and the female may get pregnant. And she 'll grow in size and give birth after 3 Sim days. The cinematic that sometimes appears has cheesy, 70's music. I turn off the cinematics, so I don't see them anymore. If things got any more graphic than that, then the T rating would be changed to M. It's your call as to whether your child is mature enough for that. There are lots of 9 and 10 year olds who play it.
On the other side of the coin, there are other aspects to the game. Some Simmers love to just build houses or community lots. To play with the Sims, you have to manage their lives. Make sure they get enough food and sleep so they go to work. If they miss work, they can be fired. Sims with kids have to keep them in similar shape and if the kids don't keep up with homework and get an F, the Social Worker takes them away. If Sims don't pay their bills, here comes Repo Man. If Sims don't learn adequate cooking skills, then they run the chance of dying in a kitchen fire every time they get hungry. If Sims don't spend quality time with the kids, the kids get really cranky and grow up with bad memories. If a Teen is really unhappy with everyone in the household, he'll run away from home. So there's a lot to keep a player busy.
If you are serious about purchasing this game, then you must check the system requirements. This game uses up a lot of RAM, needs a great video card and uses up a good bit of hard drive space.
http://thesims2.ea.com/help/detail.php?help_id=21
If you have further questions, just let me know.