More like 5/6 years older. His mom was pregnant in 2000s, (which is the reason why Harumi lost his first love interest towards her and she quit Para Para and married to young) and Kenken might had been born in late 2000 or early 2001. Gaku may had been born somewhere in between 2006 or 2007 because in 2022 he's at age 15/16, freshman in his school before he travel back to the year 2000.