How did Uchiha Obito summon Kurama?
For starters, at the beginning of the series, Kakashi was 26, and Naruto was 12. Which makes him (and Obito) only 14 when the Kyuubi attacked.

Second, presumably Obito was able to summon Kurama because Kurama was under his control, and thus acts like a sort of contract between the two. The only other person who was known for summoning Kurama is Madara Uchiha, and Kurama hates his guts.