My very good friends had a 2nd child when they were 48. Yes.
And they are VERY happy they did so. They did not want their daughter to be an "only" child. Their eldest child, their girl, is SO happy to have her little brother and she adores having a sibling. These children are 3 years apart. The Mom was an "only" child.. and she said it was the worst lonely thing growing up.. she did not want her girl to have the same experience.
For me, I had my 2nd child at 42. My children are 4 years apart. It's totally fine. We also did not want our daughter to be an only child. My girl LOVES her little brother to bits. He is now 22 months old. It now feels like a "family"... with 2 children. And yes, we did not want our eldest child to be "lonely" or to be by herself when we are long gone. My girl & her brother are truly bonded.. and they protect each other and are very "tight" together. It is real bliss to see this.
As a Parent, personally speaking.. I had to get past my "age" thing about having a 2nd child. But, in the BIG picture... it's not a big deal.. .when you are looking at the happiness and the "future" of your child to be by themselves as an "only" child, or as a sibling.... well, (in my case), you make exceptions. For me, having a sibling for my girl was very important. I was thinking about her future.. I wanted her to have a "family" when we are long gone... and to have a connection with that. I could not bear imagining my girl being all alone in this world, by herself, when we are old and then gone. For THIS reason, me & Hubby chose to have a 2nd child.
We are very happy about it. I just smile when I look at them and it is real joy.
Yes, it's not for everyone, to have a baby later in age. But this is just my experience and thoughts. I am not judging "only" children or their Parents. This is just me.
If your daughter is lonely... well, it helps having regular interaction with good friends.. but it takes time and effort to make this time for her and to arrange play-dates. As she gets older, she will have hopefully, a group of friends that will become her "friend family." She needs to be a kid... and to have socials and time with Mommy & Daddy. Talk with her.. at this age she can certainly talk about her feelings and about having a sibling...
If you don't have time for your daughter now, and she gets "lonely"... then having a 2nd child is NOT going to "cure" that "problem" for your daughter. A sibling is NOT to be had just to be a substitute "play-mate" for a child. A 2nd child is a child... and it is the relationship of siblings.. and all the time and effort that it will require.... and the time sacrifices too. So REALLY think about that.. .and do you REALLY want another baby...??????
As for your Hubby thinking he is "too" old..... well, perhaps do you know any men/friends that are older Parents? Can you talk about it with them? Really, of course you don't want to "force" him to have another child... but can he take himself out of the "reasoning" and put himself in your daughter's shoes... and think about her situation? It's not her fault that your Hubby is that age and has already had 2 kids of his own already. A 9.5 year old sibling is a big stretch in years compared to your girl...and you can't expect them to be play-mates exactly.. they are totally at different stages in their lives. But yes, being an older Parent is not something to take lightly either. It has to be thought out.
It's a hard situation... and I feel your yearning, as well as your Hubbys. But... discuss, discuss, and discuss. Think about the whole picture... your daughter too. OR, perhaps your daughter is fine about being an "only?" Just be sure you don't have any regrets... or resentments that get in the way.
All the best,
~Susan