Mine are now 18, 16 and 12. All are keen cyclists, which I tend to see as a good thing.
There is a lot going on at a young age: trees to climb, hills to roll down, balls to kick and chase. We didn't put any of ours on a bike until they were nearly four.
If it's fun, it's fun and they'll do it. One sees rather too many parents in parks trying to get their offspring into something they're clearly not interested in.
I don't think any age is 'too young' but I know lots of very keen cyclists who simply CANNOT UNDERSTAND why their children do not love bikes as much as they do. Everyone else can see why, but the parents cannot.
When they're tiny (two, three, four) it's just another fun thing to do. If it keeps being fun, they'll keep it up.
Scooters are a good way in.
Balance Bikes are fine, but you can just whip the pedals of a normal bike and drop the saddle.
I hope the above makes some sort of sense.