Are u from the UK or the US. Reason why i ask is cos i only really know about the UK ... so i'll blabber on about how it works in the UK an hopefully it will be of some use to you lol.
Having your own children will not make a blind bit of difference to adopting. In fact my SW tld me it would fair better as you have some experience of being a parent, and having a sibling for the adoptive child will be good for it. But your childs needs will be taken into consideration also. They wont want you child to feel left out in any way etc.
I have 2 grown up DD's (21 and 19) one whom lives at home (19 yr old) and the other has just moved out. The 19 yr old who lives with us will have to go thru this adoption process just as much as us, cos she is a 'grown up' so she will be interviewed, and have the police checks etc. And cos the adoptive child will be living with my DD as a sibling and regardless of age, everyone living in the same household will be interviewed. (Obvisouly your DD is a lot younger, so there wont be police checks etc)
As for fertility tmt, they do like u to have been tmt free for at least 6 mths. The reason why they do this is they have to be sure that u have finished tmt and accepted that u wont ever become pregnant again. They need all ur focus on the adoptive child. He/she is the priority for them and they have to be sure.
We had our 4th ivf tmt last Aug which sadly failed (our 3rd resulted in a pregnancy but sadly no hearbtbeat @ our 8 weeks scan - devastated) after having the 4th we decided enough was enough. I couldnt go on no more, its so drainng and heartbreaking when it doesnt work. Also had to pay for our tmt so our bank balance was crying out for some tlc too ... we turned to adoption in Novemebr, had visit from SW and a chat in Jan and we start the course in a week Fri
Now about the age thing .... there isnt a cut off age at all. Obvisouly the older u are, they prefer u to take an older child. Here in the UK most adoptive children are aged 2yrs+ because they will live with foster parents from the stage they are taken into care, and then the system tries to get them back with their birth parents etc, if this is not possible, then they go up for adoption. Very rarely u will have a child from birth, its mostly toddlers ... you can yourself deide on a cut off age, say u will accept 0-2 years, but then they wont consider u for a wee 3 yr old and that wud worry me. I think our cut off point is going to be 0-4 years. Will wait an see what happens.
Hope this is of some help to u .... looking fwd to keeping up with ur journey
BTW: DH and I are both aged 41 (42 this year
) so we are classed as 'older' parents, but hopefully - 'wiser' lol