I'm not sure if this is typical or not, but I will say that my budwing is a really slow grower. She is fed the same as the others, but she is taking her time when it comes to reaching adulthood. I've had several younger mantids mature in the time that it has taken her to reach subadult. Yours are beauties!
18 months is amazing! Sounds like you found a great method of caring for them and their setup. A few other members here have had them live that long, Digger is one, but it is uncommon, so congrats. Hopefully your get to have them around for longer still.