Spence is 21 and you can see he has the dribbling ability to worry defenders. It is not an age thing it is a style of play thing.
Players don't just suddenly become great 1v1 dribblers.
You can maybe get away with that if you have other attributes like searing pace (see Kyle Walker), great finishing (see Marcus Alonso), great delivery (see Reece James/Trent).
I don't see Sess excelling in any of those areas. He will be a decent squad player for a couple of years and then get sold to a mid-table prem team when we upgrade.