Hi Molly!
When you get a chance it would be great if you can introduce yourself in the
Introductions Forum. In the meantime, based on what I am reading here I am going to assume you are MtF (please correct me if I'm wrong).
I am MtF and I just turned 54. I know that's approaching upper years to be having childern but it's possible I might decide to in the future. So I went to a local fertility clinic (I live near Seattle) and had my sperm cyrogenically frozen.
I initially tried to do this at a different Seattle-area fertility clinic recommended by my gender therapist, and the other clinic told me they require a letter from my therapist or doctor. Frankly I was shocked and offended. I asked them, as an adult do they really think I need to obtain permission to masturbate in a cup? I took my business elsewhere.
At the fertility clinic I went to I gained some interesting information. The doctor I met with explained male sperm is produced roughly 72 days in advance - there is a brief window after starting HRT where it's possible to preserve sperm that was produced before the start of HRT. From a medical standpoint the age of a person producing sperm is less critical than age of a person producing the egg. They examined my sperm in their lab and found it was just fine from the standpoint of freezing and potential use in the future.
A more practical issue is many MtF on HRT have found they have increasing difficulty in producing the stuff you would need to get frozen. After a longer amount of time on MtF HRT sperm count drops sharply and becomes unusable. My endocrinologist also asked about this at our first meeting, her advice is freeze the stuff soon as possible if there is any possibility of wanting this in the future.