The current mutations are available only at their listed levels
The Bloodline Mutation was created specially to allow a more diverse customization of your sorcerer bloodline powers. The only restriction is that you must meet the prerequisites for that bloodline mutation:
Although heirs to similar arcane bloodlines may share commonalities, the unique circumstances in which a bloodline enters a bloodrager or sorcerer’s lineage can result in the manifestation of particularly strange or unusual bloodline powers known as mutations. Whenever a bloodrager or a sorcerer gains a new bloodline power, she can swap her bloodline power for a bloodline mutation whose prerequisites she meets. Once this choice is made, it cannot be changed, and a bloodrager or sorcerer cannot swap a bloodline power that she has altered or replaced with an archetype for a bloodline mutation. A bloodrager need not be in a bloodrage to use her bloodline mutation powers. Alternatively, a bloodrager or sorcerer can select a bloodline mutation in place of a bloodline bonus feat, provided her class level is at least equal to the level of the bloodline ability the mutation normally replaces.
Also note that when replacing a bloodline bonus feat, you have a minimum level to replace, so that was certainly considered by the developers.
However, do note that the 3 published mutations have a more specific wording that does not allow you to replace a 15th level granted bloodline power:
This ability replaces the sorcerer’s 1st-level bloodline power or the bloodrager’s 4th-level bloodline power.
This ability replaces the sorcerer’s 3rd-level bloodline power or the bloodrager’s 8th-level bloodline power.
This ability replaces the sorcerer’s 9th-level bloodline power or the bloodrager’s 4th-level bloodline power.
So, while the bloodline mutation rule certainly would allow for a mutations, in general, to replace a lower level power, the published mutations do not. This has been confirmed by the author, Alexander Augunas, as well:
When I wrote bloodline mutations, I intended you to swap what I listed as a swappable power. If you don't want to swap that power, take it as a feat.
This means that your only option is to replace one of the bloodline bonus feats gained at least at the level of the bloodline mutation you want to obtain, at 13th level, for instance.