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St. Thomas Aquinas was born in 1225 or 1227 and died in 1274. Pope Innocent III (1198-1216) had already defined the dogma:
"We say that a distinction must be made, that sin is twofold: namely, original and actual: original, which is contracted without consent; and actual which is committed with consent. Original, therefore, which is committed without consent, is remitted without consent through the power of the sacrament; but actual, which is contracted with consent, is not mitigated in the slightest without consent. . . . The punishment of original sin is deprivation of the vision of God, but the punishment of actual [mortal] sin is the torments of everlasting hell." (DZ 410) - Source: Limbo in Church Doctrine | Question from John Harden on 01-01-2008 | EWTN Catholic Q&A.
We should then expect that St. Thomas Aquinas would adhere to this Church dogma and teach it; and he does so in New Advent: Summa Theologica > Supplement > Question 69. Matters concerning the resurrection, and first of the place where souls are after death > Article 7. Whether so many abodes should be distinguished?
The abodes of souls are distinguished according to the souls' various states. Now the soul united to a mortal body is in the state of meriting, while the soul separated from the body is in the state of receiving good or evil for its merits; so that after death it is either in the state of receiving its final reward, or in the state of being hindered from receiving it. If it is in the state of receiving its final retribution, this happens in two ways: either in the respect of good, and then it is paradise; or in respect of evil, and thus as regards actual sin it is hell, and as regards original sin it is the limbo of children. On the other hand, if it be in the state where it is hindered from receiving its final reward, this is either on account of a defect of the person, and thus we have purgatory where souls are detained from receiving their reward at once on account of the sins they have committed, or else it is on account of a defect of nature, and thus we have the limbo of the Fathers, where the Fathers were detained from obtaining glory on account of the guilt of human nature which could not yet be expiated.
Summary: According to St. Thomas [in line with Church Teaching], the punishment of those dying in actual [mortal] sin and those dying only in original sin is eternal hell, though punished with different penalties and in different places. The former going to hell proper and the latter to limbo of children.
Further Reading: The Four Sections of Hell (St Thomas Aquinas) by Dr Taylor Marshall.