One of the curiosities in Bendigo's first half result was that a surge in new digital banking customers was accompanied by only an incremental increase in retail deposits.Managing director Marnie Baker spent quite some time on Monday answering questions from analysts about the bank having little to show on its balance sheet from thousands of customers flocking to Bendigo's digital startup, UP and other group brands.Baker has maintained since last October that Bendigo has been winning customers from the major banks as the horror show unfolded at the royal commission.However, the bank's retail deposit base only rose by 3.2 per cent during the December half, which appeared to belie the sharp rise in customers.In her presentation to analysts Baker said that net customer growth expanded by 57 per cent in the December quarter, noting that the average age of the new customers was much younger than Bendigo's traditional base, by 4.6 years.Most of the new sign ups are coming through the UP digital bank  - which is attracting teenagers and people in their early 20s.A short-term problem for Bendigo is that this customer segment has fewer deposits to stump up than older customers."We've been really pleased about the net customer growth," Baker said."There's been a larger proportion of customers coming from the digital bank - UP."It's a longer term strategy for us, so I would just caution against expectations that growth in customer numbers is going to have an immediate impact (on deposits) in the short term."Bendigo's early experience in digital banking might be a pointer to the deposit funding challenges that await standalone digital startup banks such as Volt, Xinja and the Cuscal-backed 86 400.If digital banking is predominantly a thing for young customers with few deposits, then the funding task for startups could be greater than some experts had envisaged.It could mean that the cavalcade of digital banks preparing to enter retail banking may have to work twice as hard as market incumbents to raise deposits because they are not likely to bring in the fatter balances of older customers.If that's right there could be some big implications for the startup sector as a whole. The most immediate could be that startups will have to source more wholesale funding than they might have expected to support loan growth and the development of their business cases.The cost of having to do that will be much higher than tapping cheaper piles of retail deposits. Inevitably, that could constrain the ability of startup banks to pitch competitive offers for mortgages and other loans.UP claims on its blog that it is now signing up 1000 customers a day.A legitimate question that needs to be answered is whether the holders of UP accounts are actually rolling all of their deposits from an incumbent bank to the new player.It could be that many are not yet glued to the UP brand and are still testing the banking platform before making a decision to haul all their cash away from an established bank.