DD spoke fluently and with ease at 18 months. Occasionally slipped up with pronouns but mostly correct and I KNOW she liked to say 'mine' from a fairly young age!
DS is 14 months and will use names to precede objects so will hold up his ball and say 'baba ball' (he calls himself baba) or pick up his sister's toy cat and say 'dd's name cat' or my bag and say 'mama bag' etc etc. But mostly things are his. He's recently picked up 'and me!' as a phrase too, complete with thumping his chest and holding out the other hand. Usually when food is being offered out.
DS is 23m and has been doing it for a couple of months. He doesn't talk as much as DD did at his age, but I think that's common in second children - you're better at understanding them so they don't have to make as much effort!
I thought it might be a little on the late side, but he seems to be having a big leap forward this month - my 3 are all teens and not much contact with other toddlers (goes to toddler group weekly).
The hiding things/actions from us, I'm happier with - suggests theory of mind is already developing nicely (DS3 has AS, so that is a concern).
Funny Zuleika, DS2 was by far the quickest off the ground with speech. We have video of him speaking in proper sentences on his first birthday - we put that down to the tiny age gap between DS1 and DS2 - 11 months!
DD's 21 months and has just started saying me and I, although not mine yet. She's very nearly fluent now but would always talk about herself and her actions in the third person, so that's only just changing now.
Debka and Pasha, things can move VERY fast once something "clicks" DS4 has probably doubled (or more!) his vocab in the past 3 weeks (since he turned 2yo), as well as suddenly "getting" sentences.
That was after about a year of making no apparent progress - he had about 20 words at 12 months and only gained 3 or 4 more words between 12 months and 22 months, something that worried us more than a little as it mirrored his brothers (AS dx) development- not the number of words, but the lack of progress for so many months. So we are very happy he has had this big jump forward.
In any case, it is their understanding of speech that's important at this young age, not the number of words they can actually say... BTW apparently at the toddler stage any consistent sound that means something is a word! ie animal noises, saying "woof" for dog or broombroom for car, gaga for gran, would all count!