If Uchiha Madara was the only one who could summon Kurama, how did Obito do it? Also, Naruto was 16 while Kakashi looked about 30, so if Kakashi and Obito were the same age, and Obito summoned Kurama while Naruto was born, he would be younger than Naruto is now.
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.