Apologies in advance if this question is a bit dumb, it's been a while since I used ANOVA tests and contrasts....

I have a data set in which participant's ages were recorded in age groups (18-24, 25-34 etc). An ANOVA showed no sig. results, which is fine. However over half the participants were <24, so I thought I should compare that group to all the others. I decided I could either run an ANOVA with contrasts or create a new dichotomous variable and run a standard t-test. Both have turned up different results and I am wondering which would be more appropriate.

As I said it has been a long while since I've used contrast testing so I may be way off here

Thanks in advance.