Hi

how old does my Thai daughter have to be before she can own land here in Thailand

No minimum age.

The land then cannot be sold , leased, mortgaged or have any liens placed on it without court permission ( not easy to get) until she attains 20 years of age.

There is no age requirement, only that she has Thai nationality.

As a minor the land will be controlled by the parents, but for thing as selling or leasing the land one needs the permisison of the court. The court will check if it really is in the best intrest of the child.

As soon as she has a birth/name certificate, she can legally own land (property and stocks) – but under guardian until she becomes 20 years old, and the land can normally not be sold or transferred by the guardian (there might be exception I’m not aware of). I bought a plot of land to my daughter shortly after she was born.

It has been known for some land offices to have a minimum age.