I think a PP said you had to have previously had a healthy birth of your own before doing surrogacy. That’s not a legal requirement. And the Law Commission were suggesting a minimum age of 18 years old, there’s lots of threads on this.
Emma (in the BBC programme) was 23 I think and was turned down by a fertility clinic (this alone is surprising) due to her age so this led to her agreeing to using her own eggs. I've no idea how this came about, this wasn't shown. It could have been her offering or in response to a "oh no what shall we do?" Q from Kevin and Aki. It may have had no change in Emma's perspective at all, but this decisions suddenly involves her son Jacob perhaps more than it did before, but he was 2 so hardly had a say in it. For Jacob I imagine that Mia was his sister when she was inside his Mother's womb, but since she doesn't live them then it's not as if he will develop a relationship with her as her brother.
I mention this as one surrogate mother in "Breeders" spoke about how she realised how it would affect her daughter, but not up until the point where she had just given birth. It's a tragic story to recounts but I strongly recommend it (on Amazon Prime).
I don't think even 30 year olds can fully necessarily comprehend their actions, as was demonstrated by Breeders, when they convince themselves and are constantly told that they are just the carrier/host/oven/extreme babysitter. It sometimes feels like a feat in mental control or extreme persuasion.
I'm currently reading Broken Bonds, a collection of accounts from surrogate mothers (and I would also recommend this). I didn't need convincing that 'altruistic' surrogacy wasn't prioritising the child either but it didn't take many chapters to see how truly awful it can be. I think it's is rather short sighted to view it as always ok if it's within a family. Doesn't it depend on the family and those involved? Whether the woman is vulnerable? Would it be ok if she had learning disabilities, or if she had lost her mother at a young age, or was an 18 year old student who didn't want to be paid but wanted her university fees paid? Or maybe she's 36 with her own family but as a result of a horrible pregnancy she doesn't consider that career change she had previous thought about or has rows with her husband as she rejects him sexually? There are so many different scenarios and it's rare to hear about them so I do really recommend reading more around it than what we see in the mainstream.