If you post a picture we could tell if they're too young to lay. Once they get to be a year or so it's very difficult to tell exactly how old they are. My 6 year olds look like my 2 year olds, you couldn't tell by looking at them that they're much older.
18 weeks is about the earliest a pullet can start laying. 25-35 weeks is far more common especially with dual purpose breeds. Your Welsummer will likely start laying after 25 weeks, but you should get really pretty dark brown eggs, so that's cool.
You can start over but you can't change basic biology, they will lay when they are ready and not before. They need to molt every year to replace old feathers and will stop or slow down during the process because it takes a lot of energy to grow new feathers and keep warm during winter.
Good nutrition is key to keeping them healthy and allowing them to lay as much as they can. Flock raiser or an all purpose flock feed can help them during the molt and cold weather. Layer feed will not encourage them to lay it only provides the calcium they need when they're making egg shells. I prefer free choice oyster shell so they can get as much as they individually need.
Don't worry they will lay when they can and they eggs will be beautiful and delicious, well worth the wait.
Most of the I raised myself from spring and they are not laying either.
