It can vary quite a lot. I also find the ages typically given as the “normal” are often a bit early. Those are typically for the high production breeds and hybrids so they might be accurate for your ISA or maybe a Leghorn but even then, it’s a general guide and might be a bit early. And for the dual purpose breeds, it’s not at all uncommon for them to not start till 24-26 weeks or later.
When you get them can also play a role in it as well. Sometimes chicks born in the fall will just wait to start laying until the following spring due to the decreased daylight hours whereas chicks born in the spring might start sooner due to the increasing light. That’s not always 100% accurate though because I’ve had fall chicks start at 22 weeks and spring chicks wait 9+ months so it’s not a guarantee but just something that can happen sometimes.
I’ve had a lot of birds over the last 5-6 years and I’ve had a huge variation in ages. As mentioned, I’ve had some wait 9 months, some 22-26 weeks, and I have pullets now that hatched in February that have already been laying for 3-4 weeks 

 interestingly, their brothers were also early maturing. Not sure what’s up with that.
But anyway, it varies a lot!
I believe my Partridge Rock was one of the ones that laid pretty late and I believe my Barred Rock was either 24 or 26 weeks. So that one should be a little bit yet. 
My Easter Eggers on the other hand have always been pretty early, some of my first layers, and some of their offspring and grand kids (EE mixes) are the early laying pullets I mentioned. So your ISA and EE will probably be first but no guarantees.