Pregnancy?

Tell her to tell her dad or you will.

Why did you agree to keep a secret from your husband? You could have just told her no, I won't keep your secret because I will not lie to my husband.

But since you didn't do that, you're now stuck with this situation. Or you can break your promise. 

i would let her tell him

You absolutely should not tell your husband. If you do, she will never trust you again. You can ask her to not tell you anything you can’t tell him in the future, but in this situation you need to treat her like the adult she is and let her have that conversation when she’s ready. It’s not like she can keep it a secret forever, so just let her talk to your husband on her own time.

I guarantee she has a reason not to tell him yet. Ask her about it and help her work through it. Don’t destroy her trust when she’s in such a hard situation and really needs people she can rely on to support her. Just as an example, her partner may be abusive and she’s afraid that if her father finds out, there will be a physical altercation between them, or her partner will take it out on her. Or her partner may not know yet. Or she’s waiting to see if she miscarries early on. 

Whatever the reason, this is HER secret, not yours. I can respect the people saying you shouldn’t keep secrets in a relationship, but this one isn’t yours to tell. Would you look at it the same way if it was your best friend instead of your stepdaughter? 

I don't keep secrets from my husband, and my stepchildren know that.  I have no desire to play, "What else haven't you told me?" with my husband.

It's not "aged."  "Aged" is about fine cheese.  It's age.

I certainly would talk to the people who raised your stepdaughter.

Try to have a conversation with your husband about his relationship with his daughter and whether she is afraid of him, for example. Hopefully that will give you a chance to tell him about the pregnancy. She might like you to tell him. If she has a partner there is always a chance of pregnancy, but at forty he should be in work and able to pay. Also he will probably be a more mature support than a twenty year  old.....

That is a very tough situation to be in.   Why is the step daughter having sex with this man twice her age?       Can you call a meeting for all of you to be there at the same time to help her break the news to your husband?      

Your step daughter needs counseling and parenting herself.   If your husband does not want to parent her, then you should step in and do the job.    Your step daughter needs direction and help.    And, you may need to give her some good advice.  

Call a meeting so your step daughter has you as some support when she tells her dad.

No secrets between spouses.

Unless it's certain types of things.

(btw...my parents stopped parenting me before I was 20)