Golf Putters Clubs

My son is 3 years old. He's done the infant mommy and me singing/art/gymboree type classes. At age 2 years, he was in toddler soccer which went way over his head (seemed geared for 3-4 year olds).

I'd like to put him in different activities, but I don't want to pressure him or overwhelm him. So I'm a little curious. At what age did you start your kids on the following activities, if any at all?

Swimming
Music Lessons
Sports lessons (Tennis, golf, soccer, etc)
PeeWee/Little League (football, baseball, etc.)
Martial Arts (karate, kung fu, etc.)
Anything else?

I have a 5 year old niece who's been in swimming and karate since the age of 3 and tennis since age 4. So at age 5, in addition to kindergarten, she's in swimming, tennis, and karate. And her mom wants to put her in piano classes. To me that seems like a lot...3-4 extra curricular activities in addition to school at age 5? Is that normal?

How much is too much (or too little)?

Thanks...

mari