Hi Stephanie, Happy New Year!
New carer here, so can't comment on the realities - but we have taken three years to get to this stage (long story!), including being assessed twice (first time was for adoption, which we were approved for, but ultimately decided to foster first). So I can only bring theoretical answers, but here goes.
Age - you're young, but not the youngest I'm aware of. Carers of any age will need to demonstrate amongst other things emotional resilience, experience with children, a support network and if you're in a relationship, the strength of that relationship.
Different agencies and local authorities will have different policies, so make sure you speak to a number of them to get the match that works for you. For example, our agency requires one of the couple is a dedicated foster carer, and cannot undertake employment elsewhere; however, I know our local authority does not have this requirement.
When considering different agencies and your local authority be sure to ask about what training is offered, both before (Skills To Foster) and once you're approved. Ask what sort of support is available (do they have in-house professionals?).
You've asked about money - while it is possible for someone to foster as a career, remember that you're working 24/7, so you'll be working for less than the minimum wage. Consider gaps between placements too.
If you're a single carer, you might only be approved for one child - and I can't imagine you'd make ends meet. As a couple, the affordability changes.
Our local authority pays a tiny fee, whereas agencies would typically pay around £400 a week for a child of the age you're looking for. That payment comprises both your fee and money to be spent on the child. Consider how you might cope with a lengthy gap between placements. It's really important that stability of the home, which includes financial security, is as steady as possible, so you will need to make sure your calculations don't include too much uncertainty or risk.
It costs nothing to have a chat with an agency/LA, and even discussions suggest now isn't the right time, you'll learn what's expected of you when you apply for real.
Good luck!