No, you cannot get high from immature plants. Most plants take from 8-12 weeks to flower. So after flowers first appear, you have another 8-12 weeks....and then you have to dry and cure. You can smoke without a cure, but it is much better if you can give at least a 2 week cure.
The plant produces THC during the flowering period, but it takes that 8-12 weeks for the trichs to mature. The only sure way to tell if they are mature is looking at them with 30x (or better) magnification. The trichs will go from clear to cloudy to amber. Most people harvest when the trichs are starting to turn amber.