> How can I threshold the F subricks without the T-statistic (like we usually get in 3dANOVA)?

Do you mean that you want to know the directionality of the covariate effect? If so, 3dLME should automatically output the effect plus the associated t-statistic.

> I have a few other covariates that are similar to age and in one case the groups
> definitely do differ. Would that be a problem?

If a covariate differs across groups in terms of its average value, that should be dealt with extraordinary caution. See pages 35-38 of the group analysis slides in the AFNI workshop for some coverage:

[afni.nimh.nih.gov]

> I am also interested in adding gender as a categorical covariate.

Treat gender as a categorical factor (not covariate -- quantitative variable -- in the traditional sense).

> If I were to do that, would I just add it using a "+" in the -model, or do the
> covariates need to be continuous? (i.e., -model "taste*status+age+gender" )

You're using the word "covariate" in the sense that it's an explanatory variable of no interest. From modeling perspective, it does not matter from the human viewpoint: an explanatory variable in a model accounts for some data variability regardless how you call it.

You can model it as "taste*status+age+gender" if you don't believe there are any interactions between gender and taste/status.

Gang