Hear is some information based upon Missouri Dept. of Conservation electroshocking surveys at Bull Shoals for walleye. They have been stocking the lake with walleye for several years and many have unique ID tags implanted in them, so they can be identified during the next shocking survey.
In Bull Shoals they have found that males begin showing up in their survey at spawning maturity after 3 years at 18 inches long, and females show up after 4 years at 22 inches.
Hope this helps. Good luck on Beaver.