The 4 month old will produce 80 years of utility if saved, for 18 years of investment: if this is a bad bargain, reproduction always is. The adult, if saved, will produce 50 years of utility for 0 further investment. This means a higher rate of return on the adult, but a higher total future "utility revenue" from the child. Depending on who's making the investment and whether that matters to you, the answer could be either way. But now that I've written it out like this, I can see your point: the adult does have a higher rate of return.
Creating things, be they X Y or Z, usually has diminishing marginal returns. You can only make so many Xs before it becomes a worse idea than Ys, and only so many Ys and Xs before Zs become better ideas.
Unfortunately, if your factory workers have aesthetic fondness for Xs, or Ys are accidentally produced by the coffeemakers in the break room, or if Bob of the Church of the SubGenius commands Zs be made rather than As, you may wind up with a suboptimal productions. In such a situation, someone may come to you and suggest that you make Zs, but you say no. B...
A few years ago, I wrote a little dialogue I imagined between 2 materialists, one of whom was for and one against abortion, centering on the personal identity question. I recently cleaned it up and added a number of references for the biological claims.
You can read it at An Abortion Dialogue.
Early feedback from #lesswrong is that it's a 'nicely enjoyable read' and 'quite good'. I hope everyone likes it, even if it doesn't exactly break new philosophical ground.