When can a thinning area be transplanted?

At what level of thinning should the hair transplant be done?

I don't think so if the hair is very thin then it can't be grafted ..

Depend on the cosmetic difference my friend. If the thinning area is still balding then you are likely to lose that hair thats being transplanted in to. Sometimes having a procedure into a thinning area can make no difference if that makes sense due to the hair going. Be prepared to have more than one procedure.

I don't think about it.

Yes defienitely it's depends upon the surgery but the most importantly by the donor hair ...

A number of factors need to be taken into consideration as transplanting into thinning areas can cause further loss. It is important for your loss to have stabilized? Are you taking preventative meds? Experience of the surgeon amongst many others. 

Speak with your surgeon, send pictures across and keep us updated.