This plant is like 1 1/4 to 1 1/2 months old and it's 16" tall.
I'm wondering if it's OK to start flowering it or not.

Yep, you can. It depends on the strain though, if its more on the indica side, I would probably wait another week unless you have height restrictions.

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go for it, put it under 12/12

How do I tell what it is?
I have two of them, both bag seed from two different bags of weed.

One has slim fan leaves and has a nice smell to it already.

The other plant reminds me of a lettuce plant with big dumbo ear leaves and it has no odor, I assume it's a shitty plant because of the lack of smell?

I have some older pictures of the plant I want to flower in my gallery but it's grown a lot since then. It's actually grown like a motherfucker in the past 4 or 5 days, dunno why.

if they are big shiny leaves, then it is a pure or damn close, Indica. I have three out of 20 that look like a weeping willow tree with these big dark green droopy leaves. but they are some of my fastest growers! and they look kind of neat next to the sativas. brings some difference to the garden. I have no idea what the yactually are but comparing them to piks on the web I' conclude they are Indicas 'cause Sativas have thinner pointy leaves. Indicas, from what I've read, produce a more 'stoner' high. Sativas generally give a more 'body' type of high. I categorize these into "Duh" and "Yes, officer?" highs one being a heavy stupid stoner high, the other one being a type that you can talk to a cop on and not show it more of a relaxed body high. Me anything is good so I keep both strains...