About 789, his father looked for a suitable marriage. The plan was to marry him to Ælfflad, the daughter of King Offa of Mercia. Offa was willing, but also wanted one of Charlemagne's daughters, Bertha, to marry Offa's son Ecgfrith. Given the sexism of the time, Charlemagne liked the idea of his son possibility inheriting Mercia, but did not like the idea of a Mercian's offspring to potentially have a claim to Frankish lands. He rejected both marriages. Charles was later married to Juliana of Ingelheim; they produced no heirs.
His father involved him in running Francia and Saxony and named him Duke of Maine. Although Charlemagne intended to distribute his lands among three of his sons, Charles the Younger was crowned (co-)King of the Franks on 25 December 800, the same day that Charlemagne was named Holy Roman Emperor.
He died of a stroke on 4 December 811, prior to his father's demise, so the kingdom would have to go to a younger son. Maybe that would be Carloman, the next in line who was renamed Pepin? We'll see.
