¿Qué significa It just doesn’t seem like you can have grown children. ?

See a translation

Sorry for my poor information. I will give you more context and information.
I have a 20 year-old daughter, who works at a company in Japan, and my 15 year-old daughter, who wants to be a nurse is having a big test to enter a high school this coming January.
I wrote about my family to my hostdad in the US. I stayed with his family when I stayed in the US many years ago.
He wrote me back telling me about him and his family too.
I almost understood all the sentences, but just this one sentence bothers me.
Please help me.
" It is good to hear that your family is doing well. It just doesn’t seem like you can have grown children. I hope your daughter passes the test and gets her wish."

And the following sentences are the letter I wrote to him.

We are OK.
My husband works as usual. One of my daughters Natsumi, who is twenty years old, works as usual though she wants to quit the job because she is having trouble getting along with her new boss. The other daughter Yuki, who is fifteen, is studying hard because she wants to be a nurse. She is getting ready for the entrance tests to enter a high school. The first test is held in January next year. If she passed the test, she wouldn't have to take other tests.
I am still working as a home care worker. I am still a bit scared of being close to my clients whenever I work because there is still a possibility that I get the virus. However, I will keep working so long as they need us.

It's getting cold in Japan, and I guess your place is also getting cold, too.
Please stay warm and take care of yourself.

I thank you very very much. All those comments you gave me make perfect sense to me.
Again, I really thank you for thinking of my prolem together. Without you, I could not get the real nuance of that sentence.
Your reply was very fast for me, and of course you should sleep at night. Did you get a enough sleep last night? I hope you did.
Actually, I think it is me who should apologize. I was able to read your reply last night, but it was late and I didn't have energy to write my reply to you at that time. Thanks to you, I was able to fell asleep happily last night.
Oh, words are not enough to expresss my feelings....
I hope I can help you someday.

Thank you for your kind words. I appreciate it.
Please stay safe and take care of yourself.