To feel disappointed DS was left out at his nan’s b’day party by his cousin?

I don’t necessarily agree with the “no 19 year old would want to play” comments because adults do like to engage in play. It’s a way to let off steam. DH often plays football with the younger kids at family parties, I’ll do some crafts with the kids and genuinely enjoy doing it. Have you seen the parents race on sports day? Video games, board games, tones of stuff for adults. I know this is a bit pedantic but I think focusing on the play element is missing the big issues which is that he couldn’t engage with either group. There’s a right and healthy way for an adult to approach playing with their younger cousins and there’s a healthy way for a teenager to engage with 50 year olds.

I really don’t think him playing with young children is going to fix anything for him.
He needs to be getting confidence in social settings with adults, a family gathering is the perfect situation.
DD is an only child, and this is likely the reason she is extremely comfortable and confident in social settings with DP and I friends, all
mid fifties, because she spent a lot of time with us and by default with adults and she would have no problem going to the pub with her uncles and aunties aged mid 40s to 60s.
What sort of social upbringing did he have ? Has he really not been exposed to adult conversation, even amongst family ?
You are not focusing on the right Issue here at all - its adult socialising he needs not that of children.

The children probably see him as too old to play young children’s games now, especially if he has left college.

Has he never had a Saturday job type thing?

Stating that being on the spectrum might explain her obliviousness is not the same as stating that everyone on the spectrum is similarly oblivious.

Either the 19 yo could have been the cool big cousin and engaged the younger ones in some games or whatever, or he should be able to talk with the other adults. In the same situation, my family and their friends have made an effort to include the awkward young adult and shown an interest in them.
My 17 yo has sometimes been a bit bored at similar gatherings when it's been much older adults and much younger children, but he would always choose to be with the adults.

This is bonkers. It’s a huge age gap for children. I am the oldest of all my cousins some by 15+ years and I used to help look after the little ones but it would be me being asked by the 50 year olds to help with them and lead any games keep them busy etc. if I tried to have a break they would hound me to play something else. Is that what you mean?

A bit offensive. I’m autistic I understand completely. I’m not sure what the OP is thinking but I don’t think it’s related to autism.

Are you quite worried about him OP and blaming this young cousin is some sort of displacement activity?

Are you seriously asking what a 19 year old would do at home alone? Seriously? I’m struggling to believe that this is real. But if it is, and you have a 19 year old who can’t be at home alone for a few hours then you’ve got a real issue.

It can feel awkward and boring to be the only young adult at a gathering, especially if you're less socially confident.

But OP, you and other parents are doing your teens a disservice if you don't teach them that sometimes it's normal to feel awkward and bored in social situations. Having uncomfortable feelings does not mean that anyone has wronged us (or that there's something wrong with us).

You noticed that your DS didn't seem to enjoy himself. Instead of searching for someone to blame (and landing on an 11 and 13 year old of all people) or encouraging your DS to see himself as a victim in this situation, validate that he was in a bit of a tough spot age-wise and help him with what approaches he could use in the future. Practicing 10-15 minutes of small talk with guests at a time then taking a break to go mess about on his phone in a side room/outside will help him develop that skill without feeling like he's trapped for hours on end. Suggest he start his small talk by joining your conversation or joining in with someone else he knows well. If people see his interested in joining conversations they're far more likely to make an effort to help him with that.

He’s an adult the same as all of the other adults there. At his age I was a parent and unless I was babysitting younger relations they’d have been busy playing with their peers.

What does he normally do at home? Can't he entertain himself? I agree he should have been there, but this suggests he can't even amuse himself at home by himself, or meet up with friends. You have not set him up well for life.

Op, there’s no way that 10 - 13 year olds would want to include a nineteen year old in their games. He’s an adult, and they’re kids, and I’m sure it absolutely didn’t cross their minds that he would be remotely interested in their games. At nineteen, he should have enough about him to be able to chat to the adults in the room, especially if he is inside with them. You have a very unrealistic view of how an eleven year old should interact with a nineteen year old - it was for your DS to go and be friendly towards his much younger cousin, not for your nephew to take the initiative.

Are you on the spectrum, OP? As I can think of no other explanation for you not realising how deeply odd everything you’re saying is. Particularly as there have now been dozens of comments explaining this to you pretty comprehensively.

Bloody hell OP, is this for real??
I have a DS, aged 18. He works and has mates the same age. He would absolutely NOT want to “ play “
with younger kids! No way, never. What a weird post! 🙈

He’s an adult. Encourage the adults to include him.

I’m sorry, at 19 it’s up to him to choose what he wants to do. He had 3 options. To play with the kids, to chat with the adults, to sit on his own. He’s 19. It’s up to him to engage where he wants to.

He does have social anxiety but he has started volunteering so I’m hoping it will help as he has no friends which was the exact same for me as a teen. I don’t want him feeling unwanted and lonely like I did.

Hoping you’ve not told him this OP? That his Mum had no mates at that age either and how you felt!? In this situation and no one here is diminishing how your son felt, but him knowing how it was for you (which maybe why you’re so triggered) won’t help. We can’t protect our kids/young adults/adults from every disappointment or rejection but we can absolutely teach them resilience and how to ensure their emotional muscles are strong.

You can't use that as a reason to make some kids who are 11-13 responsible for looking after him.

Surely he must understand he's significantly older than them?

It's a shame he couldn't speak to his grandparents but at least he had you. That's how it goes at family events. Often they're pretty boring for late teens.

Bloody hell! When DS1 was 19 he was joining the army not belly aching that he felt rejected by his 11 year old cousins.

Imo you’ve failed him tbh. You should be actively encouraging him to be joining in conversations with other adults, regardless of how old they are. He might have found them interesting. He needs some routine and structure in his life. Has he ever had a job?

He’s an adult and he or u should not expect him to being included I1 year old games . As I cannot stress enough he is an adult!

This ^

Of course a 19 year old should be able to make small talk with Aunts, Uncles, Grandparents, and even people he doesn't know at a gathering of this kind.
It might not be his most fun night out of the month, but it is not exactly tortuous. People are there to enjoy themselves and relax, and would freely chat with the birthday girl's grandson. If you noticed at any point he seemed to be being left out, you could have made a bit of an effort to re-include him.

It's really odd that you are putting the responsibility on the 11 year olds to "include" your adult son. Why didn't he go outside and join them?

The fact that he stayed inside probably reinforced the idea in the younger kids minds that he is too old for their games.

This - a 19 year old does not "play", unless you mean they were playing cricket or football etc, but then an adult man playing with younger children is neither fair or safe.