Should I use an ANCOVA test? For this data set:

I want to see if the DNA levels differ if there is food or no food and if this changes due to the number of days.
Does it make sense to use ANCOVA as the Age could be considered a Covariant? And can you perform an ANCOVA test with a covariant which isn't Nominal data?
Or should I be using a different test entirely?
Many thanks x

This sounds like a case where an interaction term will be needed.
$$
y = \beta_0 + \beta_1x_{food} + \beta_2x_{age} + \beta_3x_{food} x_{age}  + \epsilon
$$
This will tell you if the food matters (test against a model with just age) and if the food matters in different ways, depending on the age (test against a model with food and age but no interaction).
You might want to use a spline to allow for nonlinearity in the age variable, depending on your domain knowledge.
