Three different clocks
The state's clock starts at formation: the LLC or corporation's filing date. Banks and SBA lenders use it, alongside the first tax return, because that is what their models read.
The bank's clock starts at the first deposit. Revenue-based funders use it because deposits are what they underwrite; formation paperwork with no deposits behind it is not a business to them. This is the clock that decides whether you clear the six-month floor.
The license's clock, for regulated trades: a contractor's license, a liquor license, a professional registration. Some funders in those trades read it as the start date because the business could not legally have traded before it.
The cases that catch people
Formed the LLC in January, opened the bank account in July: six months old to a funder in January of the next year, not twelve.
Traded as a sole proprietor for three years, then formed an LLC last spring: usually the three years count if the deposits moved to the new account continuously and you can show both sets of statements. Send both.
Bought an existing business: the history may or may not carry over depending on whether the account and the entity continued. See the guide on funding a business you just bought.
Changed banks: the old bank's statements still count. Send them, or the new account's three months looks like a new business.
What to send so you get credit for all of it
Every bank account the business has used, back to the first deposit. The formation documents. The license with its issue date. And a one-line timeline in the cover note: formed, first deposit, license, any entity change. An underwriter who can see the whole line uses the earliest defensible date; one who sees three months of a new account uses three months.