There's no harm in starting now.
I voted this season, but here's the caveat:
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Which of the following is true:
a) None of the below apply
b) You have 2 rune pages, one filled out with all tier 3 runes for AD and another for AP.
c) You have 3 or 4 rune pages, the two in b) plus one for a tank (and possibly another page)
d) You have 5 or more rune pages, with at least 1 for each role (top/mid/adc/supp/jg)
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Which of the following best describes your role preferences:
a) You only play 1 role, e.g. "Mid or feed"
b) You play any role, but are only good at 1 role
c) You are well versed in at least 2 roles, and can avoid feeding in the remaining 3
d) You're comfortable playing any role
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Which of the following best describes your champion pool:
a) You only really know 1 champion
b) You know 2-3 champions really well, but haven't done well with any others
c) You know at least 3 champions really well, and can play comfortably well with several (e.g. 5) others
d) You've got a plethora (many) of options to choose from when picking your champion
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Which of the following best describes the games you've played thus far:
a) You've only played co-op vs AI
b) You've played mostly teambuilder and blind-pick, you've played less than 10 games of draft OR you played predominantly ARAMs
c) You've played a mix of every game mode
d) You've played more draft pick than anything else
Of the above, if you answered a) to ANY question, don't start ranked - fix it before you start.
Otherwise, grant 1 point for each b), 2 points for c), and 3 points for d). If your total from the 4 questions is at least 7 points, I'd say your ready to play ranked. There's no harm in starting at that point, as you'll have even ground with your opponents and playing ranked will only make you better. Note that I'm assuming having more rune pages and runes comes from having played more and customizing rune pages to what role you're playing and possibly what you're up against (so if you're sitting at 6 points in the above, don't go out and buy a bunch of rune pages and random runes and call yourself ready).
Hope this helps. In the end, I lost a game really badly to a silver jungler and he asked what rank I was afterwards - I told him I hadn't played ranked yet and he apologized for invading my jungle so much. I was frustrated that someone thought my not playing ranked was what allowed him to wreck my jungle that game (I just had a bad game to be honest) and that's what made me start playing ranked. Which is surprising, I started ranked because I lost so badly in a normals. There's literally no harm in trying, worst thing that can happen is you'll lose a bunch and learn how to get better more quickly than in normals.