Well, getting by the technical problems (cartilage grows your whole life, such as your nose and ears, whereas your teeth would need to somehow keep the enamel and the dentin from wearing away), I doubt you’d have much trouble hiding it. The trick would be figuring it out in the first place. Today, more and more older folks tend to look younger than in the past. So assuming you grew up normal, and then at adulthood your aging just slowed to 1/5 the current best rates, you might be considered lucky till your late 40s, maybe an oddball at 50ish, unless you naturally look older when you’re younger. (I still get carded after haircuts, and I’m not looking forward to the upcoming prostate exams in a couple years, but at my 5-year high school reunion too many folks my age looked like they were already in their 40s.)
At some point, you’d probably begin to wonder about it before others did, and you’d figure it out. If you did when you were younger, and managed to avoid going to a doctor to see why, you might have years to prep yourself. If you’re still looking 20ish as you pass 50, you might not have too much time to make plans.
Realistically, all the previously mentioned options are open to you. Even if you took the morbid route of slaying people for their identities (which I think is probably more risk and hassle than it’s worth) and got caught, how long are you going to get in jail? 20 years? Drop in the bucket, and maybe an interesting change after a couple other lifetimes have passed. More likely, you’d use a variety of things, just to see what worked out best. But as some have mentioned, it’s not like most of us really dig into our neighbors and coworkers backgrounds. If you say you’re an age, then you’re that age, and if you figured it out early enough, you’d have plenty of time to work on your forgery/hacking skills to take care of the necessary paperwork.
Really though, I think most of it comes down to how you managed to live that long. If it’s just odd genetics it might be more difficult at first than a known blessing/curse that you received. Either way, after a couple lifetimes, you probably wouldn’t even think about it much, you’d just have to stay up to date on the latest in identification needs and procedures.
I’m mixed on whether it would be fun or not, though. I mean, say the average person lives to be 70ish today. Take a typical life. 20+ years is just trying to figure things out, 20 years raising kids. Now you’re 40-50, and you’re finishing up your working years (well, theoretically) for a while, then you’re dealing with grandkids and buying plots. (Obviously, I’m not that old, yet, so I don’t know what the “almost dead” do to pass time. )
So, you’d get an extra 20 years not growing up, and after a lifetime or two, you might get bored of raising kids and leaving them when you had to. So you’d have almost 40 years of just living. Assuming you went all out learning new things, you could keep busy for a while, but it’s a compounding effect. Learn one instrument, it’s easier to learn the next (piano, then guitar, etc.) Similar with other skills, to a degree. Maybe research things, jumping from lab to lab as a young genius or something. So 500 years might be a good amount, compared to other lengths, long enough to explore all sorts of aspects of life, but not so long you wish you could die.
And, assuming will to live plays a part, if you had company, it could be a good thing, some peers who are older like yourself. Or it could be a really bad, annoying thing. It’s bad enough people hold grudges for as long as they do. Imagine 5x longer! Assuming luck plays a part (unless you have the highlander regeneration thing going for you), you’d still have to avoid all the bad things that can happen to people, and your more mobile, “undercover” life might add to the dangers there. For instance, a car accident when you’re only 110 might leave you one-armed for the next 400 years, and that might be bothersome.
Still, keeping it a secret would be easy, because you wouldn’t have a choice. You’d have to survive, and you couldn’t afford for someone to figure it out. Without a doubt, if you lived way longer than me, and I’m a researcher, you’re going to spend a good portion of that extended life being tested on.