There’s no “best age” for a kid to start riding on a balance bike. Some kids are ready at one year, while some might be ready at 2 or 3. Development is tricky, and only the parents genuinely know when their child is ready for jumping on a bike.
For the sake of giving an age, 18 months is the approximate sweet spot for starting to ride a balance bike. Anywhere after 18 months up until 3 years will be prime time to begin on the bike. After 3 years, you may want to look at buying a 14-inch bike, but the Strider 14x can accommodate that.
The best age is when they’re ready. If they’re running around the house at 18 months, go for it.