Its a difficult one.
On the one hand she is an adult and perfectly free to make her own decisions. I was married and trying for a baby at 23, had her at 24. But......I would be sad if that daughter (now late 20's) had had a baby that young herself.
I can see what I missed out on by having her and her brother so young. Building a career, travel, freedom, building a good financial buffer first, that sort of thing. I dont have a private pension for example, which didnt seem so important at the time, but now at 50 odd I realise what an issue that is.
When I look back I wouldnt say I was too young in terms of life experience or maturity, but I do think it would have been better if I had waited. But, on the basis that she is pregnant and presumably happy about it, I think you just need to keep it to yourself and go with the flow. But I do sympathise.