short internodes? is this too short or is this normal?

g'day ladies and gentz, The pics below are of my plant which is 3 weeks old from germinating. It's grown somewhat bushy with very short stems on the top leaves, leaving the bottom leaves shaded and not getting much light at all (and they are kinda wilting). Is this supposed to be normal? Can I expect the stems to grow a bit longer in the next week or so?

The last pic shows the shaded leaves and the second pic shows the length of the stems.

Looking good I wouldn't be worried. Is there much of a difference from day temps to night?

I don't have a thermometer so I can't tell the difference in temp but I keep the room air-conditioning on at 24 degree celcius all day long. I can feel at night it's much cooler so I turn off the cooling fan when lights are off, but overall the temp difference shouldn't be more than 5 degrees. Do you think it's alright to let the bottom leaves wilt and die out? they will get replaced by new leaves though yea?