While it is possible to use FOR SYSTEM_TIME ALL and aggregation, it's probably easiest (and fastest) to just join the history table directly.

SELECT
  a.*,
  ISNULL(ah.ValidFrom, a.ValidFrom) AS TimeCreated
FROM Account a
LEFT JOIN (
    SELECT ah.*,
      rn = ROW_NUMBER() OVER (PARTITION BY ah.AccountNo ORDER BY ah.ValidFrom)
    FROM Account_History ah
) ah ON ah.AccountNo = a.AccountNo AND ah.rn = 1;

You can also do this as an APPLY or scalar subquery. This may or may not be faster depending on the cardinalities involved. You should try both options.

SELECT
  a.*,
  ISNULL(ah.ValidFrom, a.ValidFrom) AS TimeCreated
FROM Account a
OUTER APPLY (
    SELECT TOP (1) ah.*
    FROM Account_History ah
    WHERE ah.AccountNo = a.AccountNo
    ORDER BY ah.ValidFrom
) ah;