Nadia I think you're absolutely right, I also think It's one thing people can say that they think is a general,rounded insult, an insult that will work, so they say it.
Ibsjob87 -I was thinking, no other animal stops 'playing' as they age, do they-until they're really old, anyway, some people just seem to assume that we should be all act a certain as we age-which is fine in essence,if they're content with it0 but to judge others who don't act a certain way, isn't.
I feel this should have been put in the Philosophy' section, now. 
is it wrong that I'm tempted to suck my thumb at anybody that calls me immature , now
chrome good for you, I do things I enjoy, too. I don't think I'd have ever questioned it apart from the word has been directed at me a fair few times in the last 6 months or so.
TobysMum yes, they do and It's one thing I find particularly irksome because as I said in my OP,I didn't really understand what the word meant!
meditrina, interesting.
Boaty okay that's my strategy from now on!
With my friend, the conv went something like
'Where have you gone?'
(next day)
'Hi sorry I was so drunk'
'Where did you go, I'm a bit upset-not see you for ages!That was my birthday night so I'm a bit upset to be honest.'
'Ah sunflower, that's very immature'.
(Turns out she'd gone to a cheap bar with another friend first, ordered 3 bottles of wine between them because of an offer that was on, and 'lost track of time' as she put it).
My Father called me immature, and also told me I 'didn't have enough real things to worry about' after I visited him a few days ago and told him I had been to the GP because of a rash I've developed on my face! Then again he's a funny 'un anyway.
Not quite the same but I was getting changed in front of a friend at the gym, and she spied my navel piercing.
'Wow you still have your bellybutton pierced!'
'Yes, have for a long time'
'Aren't you a bit old now?'
'Who says?'
(I still like it, so why not?It doesn't mean I don't pay my bills, or that I assume everyone wants to see it....)
So yes people do say it. Thing is, I'm very sensible too, sometimes-probably too sensible! Just I guess everyone's different and most of the time, I don't think 'maturity' comes into it.
Thanks for everyone's POV :)