I thought I would add a little more, further to my previous post on the youngest MP. R. G. Thorne in The History Parliament: The House of Commons 1790-1820: I – Survey (1986) provides data on MPs elected under age in the late 18th and early 19th Centuries. As he writes:
“There were at least 29, possibly 30, new Members who had not attained their legal majority (21 years) when elected to the House in this period – omitting the 15 or 16 Members who were minors when first elected before 1790 and were still in the House. Only one of the new minors, Sir Thomas Mostyn elected for Flintshire in 1796, was unseated on account of his minority”.
As he notes, the others sat despite minors sitting in the House being prohibited by law.
Three minors were elected in 1790, three in 1796, three in 1802, three in 1806, five in 1807 (one of whom was first returned in 1806), three in 1812 and one in 1818. Another 11 were returned at by-elections. Of particular note was the fact that Viscount Jocelyn, returned in 1806 as the MP for county Louth, “was barely 18 years old”.
Thorne notes that most did not vote or speak until they attained their majority, but Viscount Milton was under age when he spoke against the slave trade in 1807.
As the prohibition on minors sitting was enforced after 1832, we can conclude that Mhairi Black, the 20-year-old MP for Paisley and Renfrewshire South, is not the youngest MP since 1667, but the youngest since the early 19th Century.