I'd forgotten that devilinside, ds never once batted an eyelid if someone picked him up and took him away from me. I remember thinking he didn't love me or wasn't securely attached because I'd see all my friends dcs and nieces and nephews kicking up a proper stink if they had to be left or were passed around for cuddles. Ds2 was shy, quite clingy and preferred not to talk to people he didn't know well, but dd went through the whole screaming the place down every time I went out of sight for a millisecond thing. It was a huge shock to my system. I thought I'd done something horribly wrong, until the HV pointed out that it's a normal developmental stage. 
Ds1 is still not at all shy and will talk to anyone - even if they are a complete stranger. He is too trusting, despite knowing all about stranger danger, he can't generalise it to actually mean the people he comes into contact with. In fact it's included in his statement, because it make him very vulnerable. He went for a blood test the other day. Walked in, sat down and started telling the phlebotomist all about Nintendo and that he's getting a 3DS for his birthday. Poor woman looked totally
as he didn't even say hello before he launched into it. He would do the same with anyone he meets, on the street, in a cafe, walking along a corridor etc.
Re regression, ds didn't regress in the generally accepted way or at the typical stage, but when he hit junior school it appeared as if he'd had a massive regression. What was really happening was him no longer being able to mask and cope with the demands on him, so he shut down instead. I'm not sure I'd call it a regression, but there are still quite a few things that he's never properly caught up with ever since.
I was talking to dh the other day and we decided that, as Polter said, he has disordered development. Some people will say you should expect a child with Aspergers to be X number of years delayed, but for ds it's much more spiky than that. Academically he is ahead of a lot of his peers and in fact we've just been told he should be way ahead of most of them based on his capability/potential. Verbally and intellectually he is advanced, but his communication skills are poor and hard to assess, as his verbal skills and vocab mask his lack of understanding.
Physically he is now lagging behind his 8 year old brother and I'd probably put him at around 6/7 (he's 11 in 3 week's time). Socially, it very much depends on the situation, but he is basically stalled at the pre-junior school stage, so 7 in some situations, younger in others. Emotionally, we realised recently that he is actually on a par with - or even slightly behind dd, who is just 4. His reactions to situations are very like hers, instant, OTT and reactionary with no time for thinking things through. In fact, they are so similar in terms of emotional development that it often causes fireworks. 