Depends...
Hi alcaray
I'm assuming Triton was captured ~ 4 gya, soon after Neptune migrated to its present position as I can't see Triton or Nereid surviving all the planetesimal flybys that Neptune's migration required. The two might be younger, but we've no way of knowing aside from the fact that the number of large Neptune-crossing Kuiperoids has probably exponentially declined since migration - thus the odds are that Triton was captured long, long ago.