I post quite often on such threads and I'm in the minority view of happy to introduce early.
Frankly, when you've had a divorce I don't see how you can ever decide a new partner will be for life - even if you marry them!
So there will always be the risk that I put my daughter through another breakup. Now, some people might say no introductions til their children are adults. I've heard that said here - and it's a fine choice for an individual. But not for me.
For me, it comes down to your child's personality and how you present the new relationship to them.
My 7yo (split at 4) does not see my fiancé as a replacement for her father, in terms of her relationship with him. My fiancé and I have had family day trips with her, we're off on holiday soon, and when he stays over she likes to come in and cuddle me in bed with him there too. She likes him.
But - he's mum's boyfriend. Not her dad, or replacement dad. Children can understand different relationships. Just as her aunt is not her mum, but is a positive thing, so is mummy's boyfriend. Incidentally, she thinks the sun shines out of her father's girlfriend's arse! But she knows who her mum is.
I have always told my daughter than dating is when you decide about someone. And that you change your mind sometimes. I'm comfortable that that is a healthy thing to learn.
He's around a lot - and acts very like one of my other male/female friends, but more frequent, in that, he doesn't discipline or comfort her - that's my job.
I really think kids can tell the difference between a new partner and a parent!
I introduced her to one previous boyfriend. He still works at my house (a plumber, I have a big long term project on!). She chats to him when he's here. Last week "let me tell you the big new (XBF) - mummy is getting married!" 
Don't present your new boyfriend as their new daddy, and you'll be OK.
Disclaimer: base it on your child. Mine is super laid back and understands nothing is guaranteed, but not in a way that upsets her.