one-way repeated measurements ANOVA

Dear all,

I am new to this forum and fairly unexperienced when it comes to statistics. Maybe you could help me with a problem that I have encountered:

My experimental design is as follows: I have a population of cells, which I seperate into 3 different groups of young, middle-aged and old cells. Afterwards I treat them with a substance and measure their size every 5 minutes. In the end I have 4 groups (3 groups of different age and one where all are combined) with 7 mean values of their size (1 before treatment, 6 afterwards). What I want to do is compare corresponding times inbetween the groups and calculate p for each point in time. So for example: Is the size at 0 minutes significantly different between old and young cells.
For this I tried PRISM's one-way repeated measurements ANOVA with Newman-Keuls test. It now tells me that young and old cells are significantly different, but neither gives me exact values (only * or **), nor does it tell me at which points in time they differ.
Is it correct to use this test in this setup and if so how can I get more detailed analysis?

Thanks in advance