Hopefully they make more.

A lot of films do that. They set their sights on a comprehensive one-and-done hit, which is probably the right approach but when they get a surprise hit and the studios executives see the IP has money potential then they scramble to come up with a way to make more.

The platinum dunes Texas Chainsaw Massacre films did that with the 2003 remake. They had to turn to a prequel as opposed to a sequel, as is the case with Orphan.

They could just churn out a few more prequels that take place between Orphan: First Kill and the 2009 film. Or retcon the ending for the first Orphan film, like some old school slasher franchises did over and over again.

It doesn't matter how old she actually is, as long as she lives she will look like a 9/10 year old. If you're talking about the actress Isabelle Fuhnrman you're correct, i think they did a very good job de aging her for first kill but once she gets to 40 like you said that will be almost impossible but i don't think they will continue making films that far into the future. I think they made a sequel to the 2009 film because in it there was a lot of background for Esther that coulld be expanded upon in films, like how she got to America in the first place which we saw in first kill, what i mean is i don't think this was some phoned in rehash that you see with lots and lots of films these days given how much effort was put into it, the de aging of Isabelle, the twist and so on.