Easy, the best size bike for your kiddo is the bike that fits them from day one. The best way to do that is to measure their inseam.
Age is more like an approximate fit. Inseam is an exact fit, trust us.
Good question. Inseam is the distance between the ground and the crotch (just in case you're interested in measuring your inseam for your next Halloween costume).
We're all different heights and shapes, even when we're the same age. This is especially true with kids. Because they grow like crazy, they all have different heights and leg lengths. By measuring inseam, you're making sure that your child's bike has the perfect fit. A bike that perfectly fits means they'll be experiencing two-wheeled success and confidence faster than you can say Strider.
Easy. Have your child stand up really straight with their shoes on (you can have them stand with their back to a wall if that helps). Place a book between their legs and slide it up the wall to meet the crotch seam of their pants, just like they were sitting on a bike saddle. Measure from the top of the book to the floor and BINGO! You've got your inseam.