She may even be a solid blue. Many of them blues do have ghost stripes when kittens - even some purebred russian blue may have them. They disappear later on.
Mine guess she is 5 weeks, perhaps even 6 - she seems quite movable and agile.
Still, she is still essentially a baby to take care of alike a rescued orphan.
Yes, this is a Solid Blue kitten, I also think around 5 or 6 weeks of age (Maybe older, depends on her health on her earlier days). You can tell by her ears, eyes and mouth she is a solid kitten