Given the industry dynamics, you could easily get hired by a regional and likely upgrade fairly quickly (less than five years). You could probably even make it to a major.
45 is not too old...unless you had weak flying aptitude to begin with. After age 40 it simply gets harder to learn new tricks. The fact that you've been flying GA already means you'd probably be good to go. But only you can truly self-assess. Worst case, it doesn't cost much to give it a try...