From our experience you would say the age range that you feel happy with. As part of your assessment your social worker will explore what experience you have, and whether you are being realistic, and, if you are, probably suggest to panel that you are approved for that range.
For example, for babies, have you thought about how you'll cope with interrupted sleep - ideally you'd have experience of looking after babies overnight, not just for a couple of hours during the day. (But hey, I had no experience with babies before I gave birth to one, and I coped okay.)
If you think you'd be suited to teenagers, are you basing that on having had well-behaved nieces and nephews to stay, or have you got what it takes to support a sexually active young teen who self harms, someone who is refusing to attend school, or who pushes every boundary and has a police record?
(You may well be suited to the whole range, but some people imagine it will be like looking after their kids' friends, or their friends' kids. In some cases it will, but a lot of looked after children have experiences that can give rise to challenging behaviours, and make them harder to 'parent'.)
The approval process is very thorough, and gives you a lot to think about. We had an intense 6 session course which two couples dropped out of after the first week. And lots of discussions with your social worker. At the end of that you should have a good idea of an age range.
We're approved for 0-18, but with a preference for primary aged children, as that tends to work best for our family. But each child is an individual, and we've said no to under 10s where we don't feel we can offer the support they need, and have successfully had teens for short periods.
Good Luck with it all. It is the hardest thing I have ever done, but I love it.