Koulibaly will only be 29 when Laporte, Cancelo and Mendy are 26. He's 3 years older, it's really not a big deal.
He could play next to them for the next 4/5 seasons easily, and that's long term in football these days.
You might be right. Having Laporte and Koulibaly as our centreback pair for the upcoming 4-5 years should generate atleast one CL trophy. Would be one of the best in the world.
This is the missing piece really. Foden/Bernardo/KDB/Rodri/Ferna is considered one of the best midfields on the planet, and our attack is one of the most goal scoring happy in Europe. A world class defense is the only thing missing.