She would be 4 1/2 months old then, right? 18 weeks.. Yes that is just about right for age.
Take them off the medicated starter - they don't really need that after week 8. You can put them on a grower with oyster shell free choice. That is my recommendation, and what I do for my flock of mixed ages. I don't want to risk the harmful effects of calcium on the roosters and young ones who get into the layer food. The girls have very strong eggs because the oyster shell provides them with all the calcium they need.
Congrats on first egg! Sorry it was eaten before you got to it.