Driven by the curiosity of the word 'fenestration' used in something other than architecture, I found some web info: It seems that maturity and humidity may be factors. So one could surmise that ideal growing conditions could bring this about earlier. I know this is not the first hand experience you were seeking.

I don't remember whether it was on this board or another one, but someone wrote that the parts of either Monstera or Philodendrons that are furthest up the trees they climb on have larger leaves with more splits.

So it’s not dependent on age but on the growing conditions?

I’ve done a few searches online but I can’t find a definite answer.

"...furthest up the trees they climb on have larger leaves with more splits." This applies more to Epipremnum aureum. Shorter M. deliciosa close to ground level can have very large leaves and fenestrations. In my years of growing them I've found that cuttings taken from the same plant at the same time can vary a few years in developing fenestrations. It's more the environmental factors that control it.

Funny thing, I checked the unfurling leaves today and one of them has a hole! The others don’t.

I recently moved the plants to a more shaded location. Maybe that triggered the fenestrations? Because it’s getting less lght than it used to.

The plants are only 5 months old grown from seed.

Picture!! :)

I finally got one a month and a half ago. It gets a new leaf about every 5-6 days but has many stems. About 4-5 of the new leaves have no fenestration but 2 of them do. One small leaf with a hole is unfurling now.

I've been putting the plant outdoors during the warm humid summer and only bring it in during forecasted rain or windy conditions. I mist it most of the mornings that have no dew. I left it in a sunny spot for about 3 hrs on a clear day around noon with 3 other plants and all got leaf burn. Aside from that mishap - and because the smaller new leaves have fenestrations I am led to think it is due to these ideal shaded humid outdoor summer conditions.

Also shown is a Pothos Manjula with sun burn:

It all depends on the conditions ! The happier your plant is the more fenestrations you will have on your new leaves. This plant it’s small and only 2 feel tall, it’s currently winter where I live ( Canada ) and he’s only getting my grow light , misting , light watering and my humidifier and he is still putting out leaves like this. They need humid air and lots of indirect light not shade.

It all depends on the conditions ! A happy healthy plant will put out leaves with the most fenestrations ! It’s currently freezing winter where I live in Canada ( basement apartment ) and my plant just put out this leaf. He gets a grow light and a humidifier for 12-13 hours a day , gets mister lightly on Wednesday’s and he gets watered lightly on sundays until summer. He is still a small plant and only 2 feet tall.