I have been known to buy the odd one as an adult and try and fill it in. I have one on the night sky, which is fun to fill in as you see things. Reminds me what to look for and keeps me sharp.
I doubt I'll bother with sending it off for a certificate tho.
I had one on butterflies as a little kid, it went everywhere with me ..just in case I saw a new butterfly or catapillar.