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There was a post on his mother's old Facebook account. July 2019. Picture of a cake with "Happy birthday Seth" on it. Captioned something like "My beloved son is turning 20 today". Sadly Rhonda deleted the account since then so there's no way to retrieve this photo. Maybe somebody made a screencap, but not me. I don't have it. 

Honestly I live for wacky conspiracy theories about Seth and Bonbi, but theories about his age are just stupid. There is no doubt in my mind that he's 20 now (turning 21 in July).