29 years old isn't old. And I'd say in 29 years you've gained lots of relevant experience, whether you realize it or not. Don't sell yourself short. In your post up there, you just described personal setbacks, perseverance, overcoming those setbacks, etc. That's called useful life experience from my perspective, and I'm sure you've got lots of it compared to the avg 21 year old wanna be (not saying 21 yr olds don't bring anything to the table, just that you've got nearly a decade of experience on them).
Can you effectively communicate? Any experience in sales, customer service, problem solving? If you've got any of that and common sense, chances are that you do have relative experience and lots of it. I assure you that relative experience doesn't equate to number of combat deployments, draw times, etc if you were thinking along those lines. And I assure you that a criminal justice degree doesn't matter either - unless required by the dept you're trying to get hired by.
The age thing does matter at some agencies more than others I guess. From what I've seen, a lot of agencies, mine included, require 20 years of service to be fully vested in the retirement system (unless medically retired), but you can't draw from your pension until you hit age 50.
Generally speaking, last I checked, barring certain exceptions for qualified veterans and .mil types, federal LE applicants have to be 35 (by date of hire or application - can't remember). Most state and local agencies, again, in my experience, seem to have 40yo cut offs. Some don't have any age limitations except for state codified age limits on LE certification (eg, mandatory badge hang up within the state at age 65).
There's a major hiring crisis in LE right now, nationwide - just like military recruiters are experiencing, for some of the same reasons. Some different. When I took the written exam for the dept I'm at now, almost fifteen years ago I believe, there were almost 400 applicants elbow to elbow in suits and ties. I'm told the last written exam they gave a month or so ago had 13 applicants. Thirteen. Crazy compared to almost 400.
So, if you're asking about whether or not you can get hired, or whether or not you should start the career, those are two different answers for two different questions. I think I've posted elsewhere on here that I'd tell my kids not to do it, but, I heard the same thing when I went through the hiring process and didn't listen. So far so good I guess.
If you've done your homework and you think you're able mentally, physically, emotionally, etc and you wanna try for twenty...heck no 29 isn't too old. Not even close.
As for what would help your chances, stay in shape, study for the exam, try and make connections within the dept (ridealongs etc), and try and ace every test you're given through the hiring process. Lots of info on that elsewhere on here too if you poke around a bit. Oh, and sit down and type up a resume/CV - if only for yourself - and put on paper how your life experience will translate to police work. I almost guarantee you that at age 29 you've got what you need. That knowledge and how to convey/articulate it in front of a hiring board will really help you out and pay off. Best of luck to you.