In the Leadership election, anyone who is currently a member, or joins/rejoins by 16:00 (local time, ie GMT), 31 October 2007 will be entitled to vote. The deadline is for the membership form and fee to be at the Membership department in Cowley Street; if you want to join at the last minute, then do so online.
Confusion has arisen because elections conducted under the rules of the Liberal Democrats in England only grant eligibility to vote to members over the age of 10 who have been members for at least a year and who have renewed their membership at least once. This rule is an anti-packing measure, and applies to the elections of officers and executive committees of Local Parties, to elections of candidates for the House of Commons and the European Parliament, and to elections of Regional Conference representatives. It probably applies to elections of candidates for principal and parish councils and to elections for officers and committees of branches, but that has not been tested. It certainly does not apply to elections for Federal Conference representatives, or to the elections of Leader and President, both of which are conducted under rules established by the Federal Party, and not rules established by the State Parties.
Neither Scotland nor Wales have implemented such rules, so elections conducted under their regulations allow all members to vote, as do elections for officers and executive committees of the three non-GB local parties (Northern Ireland, Hong Kong and Brussels-and-Europe), which do not elect candidates for public elections for obvious reasons.
Finally, Associated Organisations (and Specified Associated Organisations) each have their own rules for eligibility to vote in their internal elections, but I'm not aware of any applying a minimum age limit or a requirement to be a member for a specific period; obviously LDYS has a maximum age limit.