The target age for a thriller would be ranging from sixteen to about twenty-four, as this has been known to be the range that goes to see the most thrillers.
Something else to consider is the gender of your audience. You need to think about what will appeal to both the male and female genders. The general look on it would be that men enjoy watching thriller's that are gory, horror, contains lots of fighting and action scenes. Whilst women are more into psychological thrillers and pay more attension to the story and getting inside the character's head.
Another thing you should consider in a thriller is the actors that star in the film. People are more likely to go and see a film with a big named star rather than someone who has appeared in a film or two. People will also be more likely to be attracted to films that star their favourite actors/idols. Gender also comes into this again, if you were to star someone such as 'Taylor Lautner' then your more likely to get an audience filled with females. And equally if the main actor in your film is 'Angelina Jolie', your likely to have an audeince full of males.