I would imagine they came up with the solution of pancake makeup/wig after trying other options, but I think that doing those techniques just emphasize that he is older. I'm not sure why they didn't just use actors who were Platt's contemporaries, so it's not so obvious. In Glee, for example, Cory Monteith was 26 or 27 when the show debuted, and he didn't look any more like a teen than Platt does. But except arguably for Lea Michele, none of the cast looked like they were in high school, and they essentially ignored it. With DEH, the other main teens look younger than Platt does, so he sticks out more.