That does make sense - I like it ?
I think it's also true(ish) that 1 full production series is a little under 1 year. Steven is 13 is series 1, then turns 14 in series 2. Series 2/3 are kinda the same series based on production numbers IIRC (but got split up by the network). Ditto 4/5, I think.
So that'd mean the whole programme (S1-5) takes place over about 2-3 years, with S1 = Year 1, S2-3 = Year 2, S4-5 = Year 3. So on average one episode would represent up to about a week. Depending how much time passed between the previous episode to it, and from it to the following episode.
That said Steven only has one birthday story, so maybe it represents just two years. Or maybe they were too busy to celebrate. Or maybe Steven decided being a 'large man' meant giving up on birthdays ? Or maybe I'm talking rot, which is also a thoroughly plausible explanation!