I think when women who had a child very young, give advice like “wait until you’re 32 and have a degree”, it doesn’t mean that they regret having their child, but that they are imagining what it would have been like to have had that child when they were a bit older, even though logically the child wouldn’t exist if they had. I think she has both of your best interests at heart.
I think you ought to consider this as a possibility.
She could well be projecting what she wishes she'd done when talking about you waiting until your 30's.
I know it must be daunting when considering how/when to tell her.
It's not just fear of her reaction I suspect, but also the potential of having someone pour cold water over an announcement that you and your DP are happy and excited by. In that regard it's tempting to keep it a secret for as long as possible to stay in your happy bubble.
I've thought quite hard about this and overall I think telling her sooner rather than later is best.
My rationale is that if she is really disappointed (and will articulate this) then she will be so whenever you tell her.
On the other hand, even if it's not what she would have chosen for you and your DP but when confronted with the knowledge of an imminent grandchild she might surprise you and any issues/anger might stem from being kept out of the loop.
I think it might be possible to wait for your 20 week scan and say you both (white lie) had agreed not to tell anyone before that (which if she has guessed gives a good reason for not telling her before) - if she knows your medical history (it's not clear if she does from your posts) then that adds further legitimacy to this explanation.
I wouldn't however wait until you move out.
You're very slim from what you've posted and your pregnancy may be far more obvious than you realise and will only become more so.
The last thing you want is to be put on the spot and either have to lie or tell her whilst feeling on the back foot.
So overall I see little reason in waiting to tell her now to be honest but the latest would be the scan.
Finally, congratulations on your pregnancy and yes you both might be young but you sound like you've both done well for yourselves and seem to have a very clear set of goals/ambitions/work ethic beyond your years which is to be admired 