if the seed germinated, it's not the age.. either a since this is the 2nd set of leaves defintely opens possiblity that this happened after sprouting - a drip of fertilizer can fry a nascent leaf. a chunk of soil stuck where that leaf was growing etc. Could also just be a genetic issue with the plant itself. Defects are more common than people think.
if the nodes are directly on top of each other, try reducing light - temporarily. Seedlings always look slow, but once that leaf surface area increases growth can ramp up quickly. Add in a leaf essentially missing (as far as surface area) and that slows it down a bit further for sure.
if on a timeline or have space.. pop another.
tip for future especially if seed is free or abundant... plant 150% of what you need.. for just a couple plants pop 4, because why not. This way when a defect pops up you can remove it and not be inconvenienced by bad luck.