Talk:The Answer/@comment-508329-20151231071255

Steven was 12 at the start of the series.

Evidence:


 * The birthday cake in Steven's Birthday (Episode 75) says 14.
 * Onion Friend (Episode 65) takes place in July, so Steven's birthday has to be some time in the summer (or maybe early fall).
 * Maximum Capacity (Episode 43) takes place around the New Year's Eve midnight fireworks show, so most of Season 1 takes place in the previous year.
 * Ocean Gem (Episode 26) takes place on the 1st day of summer (around June 21st), which means that the first 25 episodes of Season 1 take place in spring.

So Steven's 13th birthday happened off-screen at some point between Ocean Gem (episode 26) and Horror Club (Episode 41, which clearly takes place in fall).