Senior discounts are based on the assumption that this age group is likely to be formed by people who are retired. Similarly, student discounts are intended for full time students that don't have the "expected" source of income that is having a job. I guess Tinder would be ok if they offered student discounts (or senior discounts ;) ). The issue here is whether they can charge 27 year old professionals a different rate than say 30 year old professionals.

That is not a discount for people without a steady source of income. That is potentially a deliberate attempt to skew the demographics of their user base towards younger users (although less of an issue that it would otherwise be, since Tinder without-premium-features remains free to use for 18 year olds, 40 year olds and 109 year olds).