The age requirement for the president is part of the US Constitution and would require an amendment to pass.
Article II Section 1:
No Person except a natural born Citizen, or a Citizen of the United States, at the time of the Adoption of this Constitution, shall be eligible to the Office of President; neither shall any person be eligible to that Office who shall not have attained to the Age of thirty five Years, and been fourteen Years a Resident within the United States.
There is no official time limit on how long this can take. Historically, the fastest amendment was the 26th, which took 100 days, while the slowest was the 27th, taking more than 200 years. Wikipedia has a nice table which includes the timing of amendments in the past.
Practically speaking, the amendment process is complicated enough that an amendment is unlikely to pass quickly in order to help a particular candidate.
In the specific case of the Democrats trying to do it right now in 2019, it's not possible. Proposing an amendment requires two thirds of both houses of Congress or two thirds of the legislatures of the states. Democrats do not control two thirds of either house, nor do they control two thirds of state legislatures. This means that Democrats, acting alone, cannot propose any Constitutional amendments at this time. Actually ratifying an amendment requires three fourths of the state legislatures or conventions in three fourths of the states, an even higher bar that likewise won't be met. Neither party has the kind of supermajority required to pass amendments that the other party doesn't support.