Depending on the art, age, and the way that they became tenth dan, it kind of depends. In kendo, the highest dan grade was ninth until a year or two ago when the FIK stopped issuing ninth dans, so the highest is eigth dan.
The level of depth in the art and the sublties that are mastered are staggaring by that level. Also, one has the perspective of age and the ability to see the big picture in a way that a fifth dan is not able. In arts where an association test is required, an eighth to tenth dan cannot promote a fifth dan to sixth on their own. But they can probably help that same fifth dan to find the ways to improve in the art and to attain greater depth and to prepare them for their sixth dan testing.
Now, if you are an eigth to tenth dan who is in their thirties, this will not be the case, which begs the question of how one got to eighth or ninth dan at so young an age. Many systems have minimum ages for high dans as well, so most eighth to tenth dans in their thirties or probably even forties, are likely not true eigth to tenth dans.
Chances are, a ninth dan under the age of forty has probably started his or her own federation and/or art declared himself or herself ninth dan, in which case they really are only whatever dan they were prior to that. Or they were promoted by someone else under questionable circumstances.... or simply registered themselve with an organization that takes a fee and does not check credentials.
Now, in arts where tenth dan is posthumous, it means that they have the answers as to what comes after death and are presumably continuning to train.
Daniel