Ended up having a bit of a ramble about this on another thread and it seems it is probably relevant (more relevant?! actually relevant rather than me just waffling?!?!

) here.
I think although someone who started an instrument as an adult is undoubtedly an adult learner, if one already plays something else (especially to a high level) then it is quite a different situation to someone who is new to music. There are challenges we share in common (especially things like being takent seriously and finding people to play with and for!), but I can't help thinking that our experience of being adult learners will probably be quite different.
Even if one is learning an entirely different instrument, with a new clef, the knowledge, experience and other advantages an advanced musician brings to a new instrument is simply enormous. Sense of pitch, sense of pulse, sense of rhythm, wide knowledge and appreciation of music... seriously, I could go on and on! These are all things that most beginners will take a while to learn and which on the other hand many advanced musicians will find as natural as breathing - as children, those skills may actually have taken longer to learn, but now, they are ingrained, sometimes almost unconscious.
Some instruments transfer skills better than others, of course, and each person will have instruments they do or don't "click" with, but even so, the already acomplished musician has a wealth of knowledge and experience to help them that the beginning musician can (usually!) barely dream of having.
Learning a new instrument and being a beginner again can be immensely frustrating, and has pitfalls and difficulties all of its own, but those of us who have a good deal of music under our belts so often have an easier ride in so many ways than children and adults who are genuinely starting from nothing. (Those who teach or play full time for a living also have another disadvantage in amongst the advantages of course, in that music practice can end up feeling like an extension of work, not an escape from it!)
I'm very grateful for that and I have to acknowledge, as I have before, that adults especially who are learning entirely or mostly from scratch get huge, huge admiration and respect from me for that - I can't imagine what guts and determination it must take when one has no musical background at all and has to start out from the musical equivalent of "A is for apple...". I feel quite awed at the dedication so many adult starters put in.
It's the one thing that sometimes makes me feel a bit of a fraud describing myself as an adult learner. I often try and stretch my musical wings, but the language, aurally and theoretically as well as practically, of music is something that's been part of my life for 24 years. It's an enormous advantage, and I can't ignore the fact that it makes life appreciably easier for me - it would be disingenuous to downplay it or make out that it was a level playing field.
The only analogy I can think of is when several 4th year students in my year (I was among them), having just returned from a year in Russia, took a class in beginners' Croatian (a related but in many ways quite different language) with a group of 2nd year students (most of whom had had only a year of Russian). There was no getting away from the fact that, although we might all occassionally slip into Russian instead of Croatian, or get muddled between Cyrillic and Roman scripts, or muddle grammar between the two languages, overall we had an enormous advantage; not least that we just weren't phased by speaking and writing in a new language. The class was hugely easier for us in many ways than it was for the 2nd years.
I think we all fall at different points along this continuum, from those who are utter beginners, to those who, for example, sing in an amateur choir where they may have picked up something, all the way along to highly accomplished performers and teachers - there isn't a cut off point. We're all at different points, and we all have different pros and cons. We're all adult learners, but I think it's no bad thing to acknowledge that that term encompasses an enormous range.
I don't think I will ever get to the stage of "adult learnt", no - I can't imagine being at a stage where I don't want to get any better! So yes, in that respect I am and will always remain an adult learner. But I do think that those of us re-starting, or starting a new instrument having become accomplished on another, face a different set of challenges and have (in many ways) a lot of advantages over those who are completely new to music.
Whether this entirely makes sense I don't know, I'm a bit tired! But yeah... these are my musings about adult learners in all our many hues...