They are the same age.
For example, the tea party were their first social event. So both are at the age.
Still weird that a few years can just "pass by" considering there's a succession cold war. King's concubine is pretty lenient, going this long without any moves and not telling her son to at least act the part.
Likely timing. They still need to wait for a chance to put things in momentum.
Although, yes, the Concubine not putting leash on her son is quite stupid. She's probably overconfident about her faction's strength and the terms of engagement will be enough to play her cards.
Evan and Einrich are all still considered underage. Evan is the
first prince, he is still not declared as
crown prince. Although being eldest son, and from the main wife, he got big advantage.
Thanks to Evan and Serena's interaction, the wait-and-see faction are likely waiting for a trigger. Maybe when rumours begin about the King going to appoint one of the princes as his heir, things will finally set into motion.
Because the Violets are ranked lower than royalties, they can't ask to cancel the engagement easily. After all, it's them who accept the offer. Unless there's something really problematic they can have casus belli to complain, and have other nobilities backing them. Seems like right now, Einrich's womanizing habit is still not enough.