I have experimental data from 4 experiments with 24 subjects divided in two age groups. I want to fit the data with a nonlinear mixed regression model. I have two difficulties: how to specify that the parameters of the model should be real positive numbers and how to specify that the parameters could depend on the age group and on the experiment.
You need to describe your model in more detail. What are you trying to find out? What, if any, is the relation between the different experiments? What is the nature of your data? Once this is known, it will be possible to give meaningful answers.