"30+" is the non-alphabetic equivalent of the written phrase "thirty-plus", which is said just as it looks. Some examples of the written phrase:
The class sizes were thirty-plus (as opposed to fifteen to twenty at Abbey Hall) . . . .
—Ben Elton, Meltdown, 2009
It was thirty-plus years ago but it was still fresh in his mind since the media continually reminded the public that the last cup victory was thirty-plus years ago.
—Rick Ferguson, The Ghosts in Maple Leaf Gardens, 1998, 2013
My Quest to be a Single Dad: Thirty-Plus Years Trying to Adopt
—title of 2009 book by Garry White
There were thirty-plus people dining with us for Christmas dinner.
—Sara McDaniel H. Kemp, From Dawn to Dusk, 2013
The exact meaning is intentionally a bit fuzzy, but seems to be an ellipsis for "thirty plus [some amount]" where the amount is often greater than zero and less than ten (so "30+" equates to "somewhere between 31 and 39, inclusive") but can also be zero at the low end and/or any number at the high end (i.e. "30+" is "at least thirty"). Examples of this latter usage are easiest to see when looking at known minimums, such as the US drinking age:
[F]ans typically have to be twenty-one plus in the United States to get into the venues where bands play . . . .
— David Buckingham, Youth, Identity, and Digital Media, 2008 (snippet view)
It was a twenty-one-plus show, which worried us, but we drew one hundred and sixty people . . . .
—Jon Ginoli, My Life in Pansy Division, 2009
In both these examples, "twenty-one-plus" clearly includes individuals who are currently twenty-one-years-old, as well as thirty-, forty-, or hundred-year-olds. Many other similar examples of "X-plus" being used for "X and up" can be easily found, as in movie rating ages. And just for fun, here's a similar usage on StackOverflow:
As currently written, this will insert a hyphen between the first two groups of four in any eight-digit-plus number.
jonrsharpe's answer to https://stackoverflow.com/q/28396120