Activating JR Pass

I’m traveling to Japan in May and will be spending 4 days in Tokyo, 4 days in Kyoto, and 3 days in Osaka, a day trip to Hiroshima, then traveling from Osaka to NRT airport . I want to purchase a 7 day JR pass but do not want to activate it until I’m headed to Kyoto. I’m wondering if I purchase the JR pass in advanced, do I need to activate it as soon as I arrive in Tokyo or could I wait to activate the pass when I’m on my way to Kyoto? Should I wait to purchase my JR pass when I’m in Tokyo headed to Kyoto or would this be an issue? I’ve read various forums & websites stating that I need to purchase the JR pass in advance so the information that I’ve been researching has been a bit confusing.