The word OLDish spelled out in colorful letter candles, which is a nicer thing to put on a cake than a number.
Number candles force a decision nobody wants to make out loud. This sidesteps it entirely and gets a laugh instead, which is the actual point of a milestone cake. The letters are bright and mismatched, so they read as playful rather than pointed, and they photograph well against any frosting color.
The full display runs about six and a third inches across and just under three inches tall, so it fits a standard eight or nine inch cake without crowding. Use it for a thirtieth, a fortieth, a fiftieth, or for anyone who has started making a joke of their own birthday before anyone else can.
It also works on a sheet cake at an office party, where the number would be a bad idea.