I think this question misses the point about the 1918 Representation of the People Act. The voting age had always been 21; the issue had been the restrictions based on property ownership.
I find Wikipedia quite unhelpful on this subject. What follows is my summary of what I've been able to find out, mainly from these two pages: A Timeline of British Voting, on the website of Idox Elections ("End–to–end electoral management solutions for election professionals" – whatever that means) and Getting the Vote, in the UK Government's National Archives.
Before the Great Reform Act of 1832, only the rich were entitled to vote. You had to be male, you had to be aged 21 or over, and you had to own property worth over a certain value. A survey conducted in 1780 revealed that the electorate in England and Wales consisted of just 214,000 people – less than 3% of the total population of approximately 8 million. In Scotland the situation was considerably worse: in 1831 a mere 4,500 men, out of a population of more than 2.6 million, were entitled to vote. That's less than one in every 500 people, or less than 0.2%.
The new industrial cities, such as Leeds, Birmingham and Manchester, did not have a single MP between them, whereas 'rotten boroughs' such as Dunwich in Suffolk (which had a population of 32 in 1831) were still sending two MPs to Westminster.
By the late 18th century however, pressure for parliamentary reform had begun to grow rapidly. Significantly, much of the pressure came from men who already had the vote. Landowners were angry about the use of patronage at Westminster; manufacturers and businessmen were keen to win political influence to match their economic power. The issue began to reach a wider audience, particularly after the French Revolution and the publication of works such as Thomas Paine's Rights of Man. It was in this atmosphere that Henry Hunt addressed a crowd in Manchester, in August 1819 – an event that turned into what became known as the Peterloo Massacre.
The first (or Great) Reform Act, of 1832, gave the vote to men who occupied property in towns with an annual value of £10. This enfranchised tradesmen such as shopkeepers; but it still excluded six out of every seven adult males.
Following the second Reform Act (of 1867), 40% of adult males were entitled to vote. Seventeen years later, the third Reform Act enfranchised all male house owners in both urban and rural areas. This added 6 million people to the voting registers, but still fell some way short of introducing universal manhood suffrage.
Meanwhile the Ballot Act, of 1872, had introduced secret ballots, making it far more difficult for voters to be bribed or intimidated.
The Representation of the People Act of 1918 removed practically all property requirements for men over 21, and it also allowed women over 30 to vote. Finally, in 1928, the Equal Franchise Act removed all property requirements and gave women equal rights to men. Everyone aged 21 or over was now entitled to vote.
The voting age was lowered to 18 in 1969.
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