Certainly Christopher Monck, then known as the Earl of Torrington, was elected to the House of Commons in January 1667, about seven months short of his 14th birthday.
But he is the youngest MP we know about, rather than the only one under 21. In fact, it’s already been pointed out that several rather high profile MPs in the 18th century were elected well under the legal age of majority, at 21, including Charles James Fox, elected for Midhurst at a mere 19.
The youngest MP before Mhairi Black was almost certainly Robert Jocelyn, Viscount Jocelyn, who was only just 18 when returned for County Louth in the general election of 1806 when a suitable candidate had not been lined up.
He gave up the seat at the election the following year, only returning in 1810, after he had attained his majority – an indication of the fact that his election was indeed regarded as a bit improper.
So Mhairi Black is in fact the youngest MP since 1806.