All light horses, including Walkers, are not physically mature until six years of age (plus or minus a couple of months).
If you don't believe me ask Dr. Deb Bennett. She has done extensive research in this area.
The legs are the first to stop growing and maturing; the back is the last. That has serious implications for early riding, competing, etc.
I, personally, don't understand why folks even ask the question "how big will my horse be when it grows up?" The answer is always "it will be as big as its DNA says it will be unless humans or natural circumstances do something to screw it up." Obsessing over final size seems to be a popular, if ultimately futile, enterprise.