I took ‘middle aged’ to be something in the mid-30’s, though I guess that could be ‘young adult’ as well. In any case, I’d want to be in my mid-30’s…old enough to be taken seriously, but young enough to still have fun.
At this point, I’d settle for mid-40’s for that matter. 
-XT
The classic “best age” for women is 27, and 35 for men.
Another vote for “30-35, and none of the answers covers that very well.” For all the reasons already discussed above (in a nutshell, old enough to be taken seriously and avoid the draft, young enough to get laid… :D)
Late 20’s, early 30’s. Of course, I’m probably saying this because I’m 23 and omg i totz wanna b young 4evah, but, well, there you go.
One possible drawback of having a young appearance (late teens, early 20’s), is getting carded. If my honest-to-goodness birthdate were, say, 1901, there’s no way the clerk would believe me if it was 2005 and I still looked 25. I’d get accused of using a fake license and I’d probably be arrested or something, which would be a huge pain in the ass. So you’d have to change birthdates on your license every 30 or so years to avoid the hassle.
And I don’t really see any reason to keep your immortality a secret. Even if the person you told believed you, who’d believe them if they tried to tell anybody else?
Young child would be fun for a while, but for eternity I’m going to have to go with your ridiculously broad ‘young adult’ category.
Mid-20s to early 30s. I disagree with the assessment that you’d need to move more often, if you’re trying to hide your immortality - I’m not even immortal, and despite that, I was mistaken for a teenager a year ago - so I can, depending on circumstances, look 15 or 16 years younger than I am. Change your appearance periodically, and you can make yourself look older or younger than your base appearance - so you can get 20, or even 30 years of not even getting ‘you look older/younger than you are (claiming to be)’, and another couple decades of ‘you’re very well preserved’ on top of 5 or so years of ‘you’re that young? Really?’.
Kid or old would be fun, occasionally, to get people to underestimate you, and thus surprise them. But there’s a lot of problems with it outside that, especially if you want to have a romantic relationship.
Later middle age - 50 or so - because that’s when we men tend to acquire gravitas, but we’re still able to have fun with the ladies.
I’m another one saying mid-to-late twenties; I clicked ‘young adult’ because that was the closest fit. If I were male, I might choose to be a little older.
I’ve always thought that people are born a certain age, IYSWIM; some five-year-olds are definitely middle-aged and some middle-aged people never seem to get out of their teens. I was born 26.
I’m pretty sure we’re agreeing on the objectives, but disagreeing on the means… 
What is “middle aged” supposed to mean, anyway?
I’m not sure whether I should put myself in late-20s (when I would get mistaken for 20, and I did get boyfriends but that may have been linked to the location and not to my age) or in early-30s (I look like mid-30s now and people do tend to be less prone to trying to get paternalistic, which is nice). So I guess I’ll have 30 for 400$, Alex…
I voted young adult, say about 25 years old. I’d need to be in good shape to fight off the others. There can only be one.
Mid/late 30s. Because I think that age has a lot of flexibility to how old you choose to look - at least for a woman. Dress young, you can pass for 25 - the sagging hasn’t hit the face to give you away as older. Dress a little older and add a little grey to your hair, you are suddenly a “well preserved” mid-40s. This allows you to do the 20 year age in one spot before moving thing.
I’d also choose to be a redhead or Asian. Its hard to get a good age for most people off redheads and Asians. They just don’t seem to physically age on the same path.
Probably my current age. I’m 39 and I can be pretty well invisible if I choose to be. I can dress young or dress old and never get much comment. People have a tough time guessing my age–some guess older and some guess younger.
So, 39.
27
No good answer on the poll - nothing between young adult and middle-aged adult. I wanted just “adult.” No vote.
Middle aged – say 40s. Still young enough to enjoy life, but old enough to be respected.
I voted young adult- I’d be happy to stay just how I am now. I’m 25, and people guess my age as anywhere between 16 and 30. I don’t generally have a problem with people not treating me with respect because I’m young - I act older than I am, and I demand respect.
If I’m going to live forever, I’d like to do it young and beautiful.
I voted young adult, but I’d like to be 26. No real reason, just that 26 in real life was my best year so far.
Your immortal. Why do you care? Go kick some ass.
My appearance hasn’t changed all that much between 25 and 37 so I would say late 20s/early 30s. You can pass for a lot of different ages in a lot of different situations without standing out as obviously too young or too old. Sort of like Paul Rudd.
Because you’re immortal doesn’t mean that you can’t be blown to chunks, burned to ashes or buried alive. Unless you’re the “magical” type that spontaneously regenerates body parts, is able to be reconstituted from ashes and are absolutly positively guaranteed never to be buried when a tunnel collapses or taken to the bottom of the ocean in a shipwreck.
If we are talking about THIS world and not the TV/Movie Universe, and your immortality only extends to “not aging and not being killed by disease”, then you’d damn well better be extremely careful not to go around getting shot, killed in car crashes, dismembered by Mexican drug gangs or blown up real good. Over time you would actually become very much MORE safety conscious and not wreckless like Movie Immortals.
Oh, and I’d like to see how well the answer tracks to your current age. I can understand that if you’re 25 now, you’re probably going to want to remain that age and aren’t going to say “60”. I’m 48 and I said “40-ish”. I expect I’d probably give the same answer 20 years from now, but if I was 20, I’d say 20.